some liquid will work in the cylinder. And from the movement of the piston, just like in a steam engine, with the help of the crankshaft, both the flywheel and the pulley will begin to rotate. Thus, a mechanical
This means that you only need to alternately heat and cool some working fluid. For this, the arctic contrasts were used: alternately water from the undersea ice or cold air comes to the cylinder; the temperature of the liquid in the cylinder changes rapidly, and such an engine starts to work. It doesn't matter whether the temperatures are above or below zero, you just need to have a difference between them. In this case, of course, the working fluid for the engine must be taken such that it would not freeze at the lowest temperature.
Already in 1937 a temperature difference engine was designed. The design of this engine was somewhat different from the described scheme. Two pipe systems were designed, one of which must be in the air and the other in the water. The working fluid in the cylinder is automatically brought into contact with one or the other pipe system. The liquid inside the pipes and the cylinder does not stand still: it is driven all the time by pumps. The engine has several cylinders, and they alternately come to the pipes. All these devices make it possible to accelerate the process of heating and cooling the liquid, and, therefore, the rotation of the shaft to which the piston rods are attached. As a result, such speeds are obtained that they can be transmitted through a gearbox to the shaft of an electric generator and, thus, the thermal energy obtained from the temperature difference can be converted into electrical energy.
The first temperature difference engine was only designed for relatively large temperature differences, of the order of 50 °. It was a small 100-kilowatt power plant operating
on the difference in temperature between air and water from hot springs, which are available here and there in the North.
On this installation, it was possible to check the design of the different-temperature engine and, most importantly, to accumulate experimental material. Then an engine was built that uses smaller temperature differences - between sea water and cold Arctic air. The construction of differential-temperature stations became possible everywhere.
Somewhat later, another different-temperature source of electrical energy was constructed. But it was no longer a mechanical engine, but a device that operated like a huge galvanic cell.
As you know, a chemical reaction occurs in galvanic cells, as a result of which electrical energy is obtained. Many chemical reactions involve either the release or the absorption of heat. You can choose such electrodes and electrolyte that there will be no reaction as long as the temperature of the cells remains unchanged. But as soon as they are warmed up, they will begin to give current. And here the absolute temperature does not matter; it is only important that the temperature of the electrolyte begins to rise relative to the temperature of the air surrounding the installation.
Thus, in this case too, if such an installation is placed in cold, arctic air and “warm” sea water is supplied to it, electrical energy will be obtained.
Differential temperature installations were already quite common in the Arctic in the 50s. They were quite powerful stations.
These stations were installed on a T-shaped pier deeply protruding into the sea bay. Such an arrangement of the station shortens the pipelines connecting the working fluid of the differential-dark installation with sea water. The installation requires a considerable depth of the bay for a good pabota. There must be large masses of water near the station so that when it cools due to heat transfer to the engine, freezing does not occur.
Differential temperature power plant
The power plant, using the temperature difference between water and air, is installed on an iola, which cuts deep into the bay. Cylindrical air radiators are visible on the "roof of the power plant building. From the air radiators there are pipes through which working fluid is supplied to each engine. Pipes also go down from the engine to a water radiator immersed in the sea (not shown in the figure). The motors are connected to electric ones. "generators through gearboxes (in the figure they are visible on the opened part of the building, in the middle between the engine ^ and the generator), in which, with the help of a worm gear, the number of revolutions is increased. From the generator, electrical energy goes to transformers that increase the voltage (the transformer / pores are on the left parts
building, not opened in the figure), and from the transformers to the distribution boards (upper floor in the foreground) and then to the transmission line. Some of the electricity goes to huge heating elements immersed in the sea (they are not visible in the figure). These l create an anti-freeze port.
Effect of temperature on the engine internal combustion
Most of the thermal energy is removed from the engine to the cooling system and carried away with the exhaust gases. Heat removal into the cooling system is necessary in order to prevent the piston rings from burning, valve seats burning, piston seizure and seizure, cylinder head cracking, detonation, etc. To remove heat into the atmosphere, part effective power the engine is consumed to drive the fan and water pump. With air cooling, the power consumed to drive the fan is higher due to the need to overcome the high aerodynamic drag created by the ribbing of the heads and cylinders.
To reduce losses, it is important to find out how much heat must be removed to the engine cooling system and how this amount can be reduced. G. Ricardo paid great attention to this issue already at the initial stage of the development of engine building. On an experimental single-cylinder engine with separate cooling systems for the cylinder head and for the cylinder, experiments were carried out to measure the amount of heat removed to these systems. The amount of heat removed by cooling during the individual phases of the working cycle was also measured.
The combustion time is very short, but during this period the gas pressure increases significantly, and the temperature reaches 2300-2500 ° C. During combustion in the cylinder, the processes of gas movement occur intensively, contributing to heat transfer to the cylinder walls. The heat saved in this phase of the operating cycle can be converted into useful work during the subsequent expansion stroke. During combustion, about 6% of the thermal energy contained in the fuel is lost due to heat transfer to the walls of the combustion chamber and cylinder.
During the expansion stroke, about 7% of the thermal energy of the fuel is transferred to the cylinder walls. When expanding, the piston moves from TDC to BDC and gradually releases an increasing surface of the cylinder walls. However, only about 20% of the heat saved even with a long expansion course can be converted into useful work.
About half of the heat dissipated into the cooling system falls on the exhaust cycle. Exhaust gases leave the cylinder at high speed and are hot. Some of their heat is removed to the cooling system through the exhaust valve and the exhaust port of the cylinder head. Directly behind the valve, the flow of gases changes direction by almost 90 °, while vortices appear, which intensifies heat transfer to the walls of the outlet channel.
The exhaust gases must be discharged from the cylinder head by the shortest route, since the heat transferred to it significantly loads the cooling system and for its removal to ambient air requires the use of a fraction of the effective engine power. During the gas release period, about 15% of the heat contained in the fuel is removed to the cooling system. The thermal balance of a gasoline engine is given in table. 8.
Table 8. Thermal balance of a gasoline engine
| Share in the balance% | ||
| 32 | ||
| in the combustion phase | 6 | |
| during expansion | 7 | |
| during release | 15 | |
| General | 28 | 28 |
| 40 | ||
| Total | 100 | |
A diesel engine has different heat dissipation conditions. Due to the higher compression ratio, the temperature of the gases leaving the cylinder is much lower. For this reason, the amount of heat removed during the exhaust stroke is less and in some cases amounts to about 25% of the total heat given to the cooling system.
The pressure and temperature of gases during combustion in a diesel engine is higher than that of a gasoline engine. Together with high speeds of rotation of gases in the cylinder, these factors contribute to an increase in the amount of heat transferred to the walls of the combustion chamber. During combustion, this value is about 9%, and during expansion - 6%. During the exhaust stroke, 9% of the energy contained in the fuel is removed to the cooling system. The heat balance of the diesel engine is given in table. nine.
Table 9. Diesel heat balance
| Heat balance components | Share in the balance% | |
| Heat transformed into useful work | 45 | |
| Heat removed to the cooling system: | ||
| in the combustion phase | 8 | |
| during expansion | 6 | |
| during release | 9 | |
| General | 23 | 23 |
| Heat generated by friction of the piston | 2 | |
| Heat dissipated with exhaust gases and radiation | 30 | |
| Total | 100 | |
The heat generated by friction of the piston against the cylinder walls in a gasoline engine is about 1.5%, and in a diesel engine it is about 2% of its total amount. This heat is also transferred to the cooling system. It should be noted that the examples given represent the results of measurements carried out on research single-cylinder engines and do not characterize automobile engines, but serve only to demonstrate the differences in thermal balances of a gasoline engine and a diesel engine.
HEAT REMOVED TO THE COOLING SYSTEM
The cooling system removes about 33% of the thermal energy contained in the fuel used. Already at the dawn of the development of internal combustion engines, the search began for ways to convert at least a part of the heat removed into the cooling system into the effective engine power. At that time, it was widely and quite effectively used steam engine with a thermally insulated cylinder and therefore, naturally, they tried to apply this method of thermal insulation to the internal combustion engine. Experiments in this direction were carried out by eminent specialists, such as, for example, R. Diesel. However, the experiments revealed significant problems.
In the crank mechanism used in internal combustion engines, the gas pressure on the piston and the inertia force of the translationally moving masses press the piston against the cylinder wall, which at high piston speed requires good lubrication of this rubbing pair. In this case, the oil temperature should not exceed the permissible limits, which in turn limits the temperature of the cylinder wall. For modern engine oils, the cylinder wall temperature should not be higher than 220 ° C, while the temperature of gases in the cylinder during combustion and expansion is an order of magnitude higher, and the cylinder must be cooled for this reason.
Another problem is related to maintaining the normal temperature of the outlet valve. The strength of steel decreases at high temperatures. By using special steels as the material of the exhaust valve, its maximum allowable temperature can be increased to 900 ° C.
The temperature of the gases in the cylinder during combustion reaches 2500-2800 ° C. If the heat transferred to the walls of the combustion chamber and cylinder were not removed, then their temperature would exceed the permissible values \u200b\u200bfor the materials from which these parts are made. Much depends on the velocity of the gas near the wall. It is practically impossible to determine this speed in the combustion chamber, since it changes during the entire operating cycle. Likewise, it is difficult to determine the temperature difference between the cylinder wall and air. At intake and at the beginning of compression, the air is colder than the cylinder walls and combustion chamber and therefore heat is transferred from the wall to the air. Starting from a certain position of the piston during the compression stroke, the air temperature rises above the wall temperatures, and the heat flow changes direction, that is, heat is transferred from the air to the cylinder walls. Calculation of heat transfer under such conditions is a problem of great complexity.
Sharp changes in the temperature of gases in the combustion chamber also affect the temperature of the walls, which fluctuates during one cycle on the surface of the walls and at a depth of less than 1.5-2 mm, and deeper it is set at a certain average value. When calculating heat transfer, it is this average temperature value that must be taken for the outer surface of the cylinder wall, from which heat is transferred to the coolant.
The surface of the combustion chamber includes not only forcibly cooled parts, but also the piston crown and valve discs. Heat transfer to the walls of the combustion chamber is inhibited by a layer of carbon deposits, and to the walls of the cylinder - by an oil film. Valve heads must be flat to provide a minimum surface area for hot gases. When opened, the intake valve is cooled by the incoming charge flow, while the exhaust valve is very hot during operation by the exhaust gases. The stem of this valve is protected from hot gases by a long guide that almost reaches the valve disc.
As already noted, the maximum temperature of the exhaust valve is limited by the thermal resistance of the material from which it is made. Heat from the valve is removed mainly through its seat to the cooled cylinder head and partly through a guide, which also needs to be cooled. For exhaust valves operating under severe temperature conditions, the stem is hollow and partially filled with sodium. When the valve is heated, sodium is in a liquid state, and since it does not fill the entire cavity of the rod, when the valve moves, it intensively moves in it, thereby removing heat from the valve disc to its guide and then to the cooling medium.
The exhaust valve disc has the smallest temperature difference with the gases in the combustion chamber and therefore relatively little heat is transferred to it during combustion. However, when the exhaust valve is opened, the heat transfer from the exhaust gas flow to the valve disc is very high, which determines its temperature.
ADIABATE MOTORS
In an adiabatic engine, the cylinder and its head are not cooled, so there is no heat loss due to cooling. Compression and expansion in the cylinder occur without heat exchange with the walls, i.e., adiabatically, similar to the Carnot cycle. The practical implementation of such an engine is associated with the following difficulties.
In order for heat fluxes between the gases and the cylinder walls to be absent, it is necessary that the wall temperatures be equal to the gas temperature at each moment of time. Such a rapid change in wall temperature during the cycle is practically impossible. It would be possible to realize a cycle close to adiabatic if the temperature of the walls during the cycle is ensured within the range of 700-1200 ° C. At the same time, the material of the walls must remain operable in conditions of such a temperature, and, in addition, thermal insulation of the walls is necessary to eliminate heat removal from them.
It is possible to provide such an average temperature of the cylinder walls only in its upper part, which is not in contact with the piston head and its rings and, therefore, does not require lubrication. In this case, however, it is impossible to ensure that hot gases do not wash over the lubricated part of the cylinder walls when the piston moves towards the BDC. At the same time, it can be assumed that the cylinder and piston do not need lubrication.
Further difficulties are associated with valves. The intake valve is partially cooled by the intake air. This cooling occurs due to an increase in air temperature and, ultimately, leads to a loss of part of the effective power and efficiency of the engine. Heat transfer to the valve during combustion can be greatly reduced by the thermal insulation of the valve disc.
The temperature conditions of the exhaust valve are much more difficult. Hot gases leaving the cylinder have a high velocity at the transition of the valve disc into the stem and greatly heat the valve. Therefore, to obtain the effect of adiabaticity, heat insulation is required not only for the valve disc, but also for its stem, the heat removal from which is carried out by cooling its seat and guide. In addition, the entire exhaust port in the cylinder head must be thermally insulated so that the heat of the exhaust gases leaving the cylinder is not transferred through its walls to the head.
As already mentioned, during the compression stroke, relatively cold air is first heated from the hot cylinder walls. Further, during the compression process, the air temperature rises, the direction of the heat flow changes to the opposite, and the heat from the heated gases is transferred to the cylinder walls. At the end of the adiabatic compression, a higher gas temperature value is achieved in comparison with compression in a conventional engine, but this requires more energy.
Less energy is expended when the compressed air cools because less work is needed to compress less air volume due to cooling. Thus, cooling the cylinder during compression improves the mechanical efficiency of the engine. In the course of expansion, on the other hand, it is expedient to insulate the cylinder or supply heat to the charge at the beginning of this stroke. These two conditions are mutually exclusive and it is impossible to implement them simultaneously.
Compressed air cooling can be achieved in supercharged internal combustion engines by supplying air, after it has been compressed in a compressor, to an intercooler radiator.
Heat supply to the air from the cylinder walls at the beginning of expansion is possible to a limited extent. Combustion chamber wall temperatures of an adiabatic engine
very high, which causes the air entering the cylinder to heat up. The filling ratio, and therefore the power of such a motor, will be lower than that of a forced cooling motor. This disadvantage can be eliminated with the help of a turbocharger that uses the energy of the exhaust gases; some of this energy can be transferred directly to the engine crankshaft through a power turbine (turbo compound engine).
Hot walls of the combustion chamber of an adiabatic engine provide ignition of fuel on them, which predetermines the use of a diesel working process in such an engine.
With perfect thermal insulation of the combustion chamber and cylinder, the wall temperature would increase until reaching the average temperature of the cycle at a depth of about 1.5 mm from the surface, i.e. would be 800-1200 ° C. Such temperature conditions impose high demands on the materials of the cylinder and the parts that form the combustion chamber, which must be heat-resistant and have thermal insulation properties.
The engine cylinder, as noted, must be lubricated. Ordinary oils are used up to a temperature of 220 ° C, above which there is a risk of burning and loss of elasticity of the piston rings. If the cylinder head is made of an aluminum alloy, then the strength of such a head decreases rapidly already when the temperature reaches 250-300 ° C. The permissible temperature of the exhaust valve heating is 900-1000 ° C. These values \u200b\u200bof the maximum allowable temperatures must be guided when creating an adiabatic motor.
The greatest success in the development of adiabatic motors was achieved by the Cummins Company (USA). The diagram of an adiabatic motor developed by this company is shown in Fig. 75, which shows the thermally insulated cylinder, piston and cylinder head exhaust port. The exhaust gas temperature in the thermally insulated exhaust pipe is 816 ° C. The turbine connected to the exhaust pipe is connected to crankshaft through a two-stage gearbox equipped with a torsional vibration damper.
The prototype of the adiabatic engine was based on a six-cylinder NH diesel engine. A schematic cross section of this engine is shown in Fig. 76, and its parameters are given below:
Number of cylinders ............................................... 6
Cylinder diameter, mm ...................................... 139,7
Piston stroke, mm .............................................. ... 152.4
Rotation frequency, min-1 .................................. 1900
Maximum pressure in the cylinder, MPa ..... 13
Lubricant type ............................... Oil
Average effective pressure, MPa ............... 1.3
Air / Fuel Mass Ratio ............... 27: 1
Inlet air temperature, ° С ................ 60
Expected results
Power, kW ............................................. 373
Rotation frequency, min-1 ............................. 1900
NOx + CHx emission ..................................... 6,7
Specific fuel consumption, g / (kW h) .......... 170
Service life, h ............................................ 250
Glass-ceramic materials with high heat resistance are widely used in the engine design. However, to date, it has not been possible to ensure high quality and long service life of parts made from these materials.
Much attention was paid to the creation of the composite piston shown in fig. 77. Ceramic piston head 1 connected to its base 2 special bolt 3 with washer 4 . The maximum temperature in the middle of the head reaches 930 ° C. The head is thermally insulated from the base by a package of thin steel spacers 6 with a highly uneven and rough surface. Each layer of the package has a high thermal resistance due to the small contact surface. The thermal expansion of the bolt is compensated for by means of disc springs 5.
REMOVAL OF HEAT TO AIR AND ITS REGULATION
The removal of heat by the cooling system causes not only the loss of heat energy that could be realized in operation, but also direct losses of a part of the effective power of the engine due to the drive of the fan and the water pump. The removal of heat from the cooled surface S into the air environment depends on the temperature difference between this surface and the air tand also on the coefficient of heat transfer of the cooling surface to the air. This coefficient does not change in any significant way regardless of whether the cooling surface is formed by the system's radiator fins. liquid cooling or the fins of the air-cooled engine parts. First of all, consider engines with liquid cooling systems.
The amount of cooling air is less, the more heat is removed per unit of its volume, that is, the more the cooling air will be heated. This requires an even distribution of air over the entire cooling surface and a maximum temperature difference between it and the air. In the radiator of the liquid cooling system, conditions are created under which the cooled surface has an almost uniform temperature field, and the temperature of the cooling air gradually rises as it moves through the radiator, reaching a maximum value at the outlet from it. The temperature difference between the air and the cooled surface gradually decreases. At first glance, it seems that a deep radiator is preferable, since the air heats up more in it, but this issue should be considered from an energy standpoint.
The surface heat transfer coefficient a is a complex dependence on a number of factors, however, the greatest influence on its value is exerted by the air flow rate near the cooling surface. The relationship between them can be represented by a ratio of ~ 0.6-0.7.
With an increase in air speed by 10%, heat removal increases only by 7%. The air flow rate is proportional to its flow rate through the radiator. If the radiator design does not change, then to increase the amount of heat removed by 7%, the fan speed should be increased by 10%, since the amount of air supplied by the fan directly depends on it. The air pressure at a constant cross-sectional area of \u200b\u200bthe fan depends on the second degree of its rotation speed, and the fan drive power is proportional to its third degree. Thus, when the fan speed increases by 10%, the drive power increases by 33%, which has negative consequences, manifested in the deterioration of the mechanical efficiency of the motor.
The dependence of the amount of cooling air on the amount of heat removed, as well as on the increase in air pressure and fan drive power is shown in Fig. 78. From the standpoint of reducing energy costs, this nomogram is very useful. If the frontal surface of the radiator is increased by 7%, then the area of \u200b\u200bthe flow area and the cooling surface of the radiator will proportionally increase, and, therefore, the amount of cooling air is sufficient to increase by the same 7% to remove 7% more heat, i.e., as in the example described above. In this case, the fan power increases only by 22.5% instead of 33%. If the air flow through the fan V z increase by 20% (dot and arrows 1 in fig. 78), then the amount of removal and heat Q, proportional to V z0,3 , will increase by 11.5%. A change in air flow by increasing the fan speed by the same 20% leads to an increase in the air flow pressure by 44%, and the fan drive power by 72.8%. To increase heat dissipation by 20% in the same way, increase the air flow by 35.5% (dot and dotted arrows 2 in fig. 78), which entails an increase in air pressure by 84%, and the fan drive power - almost 2.5 times (by 149%). Therefore, it is more advantageous to increase the frontal surface of the radiator than to increase the rotational speed of the latter with the same radiator and fan.
If the radiator is divided into two equal parts along its depth, then the temperature difference in the front t1 will be more than in the back t2 , and, therefore, the front of the radiator will be more cooled by air. Two radiators, obtained by dividing one into two parts, will have less resistance to the flow of cooling air in depth. Therefore, a radiator that is too deep is disadvantageous for use.
The radiator should be made of a material with good thermal conductivity and its resistance to air and liquid flows should be small. The mass of the radiator and the volume of the liquid in it should also be small, as this is important for the quick warm-up of the engine and the inclusion of the heating system in the car. For modern passenger cars with a low front end, low radiators are required.
To minimize energy costs, it is important to achieve high fan efficiency, for which a guiding air duct is used, which has a small gap along the outer diameter of the fan impeller. The fan impeller is often made of plastic, which guarantees the exact shape of the blade profile, their smooth surface and low noise. At high speeds, such blades deform, thereby reducing the air flow, which is very expedient.
The high temperature of the radiator increases its efficiency. Therefore, at present, sealed radiators are used, the excess pressure in which increases the boiling point of the coolant and, consequently, the temperature of the entire radiator matrix, which can be smaller and lighter.
For an air-cooled engine, the same laws apply as for a liquid-cooled engine. The difference is that the fins of the air-cooled engine parts have a higher temperature than the matrix of the radiator, so less cooling air is required to remove the same amount of heat during air cooling. This advantage is of great importance when operating vehicles in hot climates. Table 10 shows the modes of operation of liquid and air cooled engines when the ambient temperature changes from 0 to 50 ° C. For a liquid-cooled engine, the degree of cooling is reduced by 45.5%, while for an air-cooled engine under the same conditions it is only 27.8%. For a liquid-cooled engine, this means a bulkier and more energy-intensive cooling system. For an air-cooled engine, a small fan modification is sufficient.
Table 10. Efficiency of engine cooling by liquid and air cooling systems depending on external temperature
| Cooling type, ° С | Liquid | Air |
| Cooling surface temperature | 110 | 180 |
| 0 | 0 | |
| Temperature difference | 110 | 180 |
| Cooling air temperature | 50 | 50 |
| Temperature difference | 60 | 130 |
| Deterioration of the mode at a temperature of 50 ° С compared to 0 ° С,% | 45,5 | 27,5 |
Cooling control results in great energy savings. Cooling can be adjusted to be satisfactory at maximum engine load and maximum ambient temperature. However, at lower ambient temperatures and engine partial load, this cooling is naturally excessive and the cooling must be re-adjusted to reduce wear and motor mechanical efficiency. For liquid-cooled engines, this is usually done by throttling the flow of fluid through the radiator. In this case, the power consumption of the fan does not change, and from an energy point of view, such regulation does not bring any benefit. For example, to cool a 50 kW engine at a temperature of 30 ° C, 2.5 kW is consumed, and at a temperature of 0 ° C and an engine load of 50% of the full one, only 0.23 kW would be required. Provided that the required amount of cooling air is proportional to the temperature difference between the surface of the radiator and the air, at 50% engine load, half of the air flow controlled by the fan speed is also sufficient to cool it. The savings in energy and, consequently, fuel consumption with such regulation can be quite significant.
Therefore, cooling regulation is currently focused on special attention... The most convenient regulation is changing the fan speed, but for its implementation it is necessary to have a variable drive.
Switching off the fan drive serves the same purpose as changing its speed. For this, it is convenient to use an electromagnetic clutch, which is turned on by a thermostat depending on the temperature of the fluid (or the cylinder head). If the clutch is turned on by a thermostat, then the regulation is carried out not only depending on the ambient temperature, but also on the engine load, which is very effective.
The fan is switched off by means of a viscous clutch in several ways. As an example, consider a viscous coupling from Holset (USA).
With the most easy way limitation of the transmitted torque is used. Since the torque required to rotate the fan increases with increasing speed, the slip of the viscous clutch also increases, and at a certain value of the fan power consumption, its speed no longer increases (Fig. 79). The fan speed with an unregulated V-belt drive from the engine crankshaft increases in proportion to the engine speed (curve B), while in the case of a fan drive through a viscous clutch, its frequency increases only to hv
\u003d 2500 min-1 (rotation curve ANDunregulated drive, increases in proportion to the third ).
The power consumed by the fan at the speed level and at the maximum power mode is 8.8 kW. For a fan driven through a viscous clutch, the rotation increases, as noted, to 2500 min-1, and the frequency required in the fan power mode is 2 kW. Since an additional 1 kW is dissipated in the viscous clutch at 50% slippage, an additional 1 kW is dissipated into the heat, the overall energy saving on the fan drive reduces fuel consumption. Such a cooling regulation of 5.8 kW, however, even this can be considered satisfactory, the air consumption does not increase in direct proportion to the frequency, since the rotation of the engine and the speed of movement remains the growth of the velocity head, in addition, with an increase in air, which helps to cool the engine.
Another type of viscous coupling of the "Holset" company provides regulation of the thermal regime of the engine in addition to the ambient temperature (Fig. 80). This clutch differs from the previously considered one in that the volume of fluid in it, transmitting torque, depends on the external temperature. The clutch housing is divided by a partition 5 (see Fig. 81) into the drive disc chamber 1 and a reserve volume chamber 2, connected by a valve 3. The valve is controlled by a bimetal thermostat 4 depending on the air temperature. Scoop 6, pressed against the disk by a spring, serves to discharge liquid from the disk and accelerate its overflow from the disk chamber into the volume 2. Part of the liquid is constantly in the chamber of the drive disk and is capable of transmitting a small torque to the fan. At an air temperature of 40 ° C, for example, the maximum fan speed is 1300 min-1, and the power consumption is no more than 0.7 kW. When the engine heats up, the bimetallic thermostat opens the valve, and part of the liquid enters the chamber of the driving disc. As the flow area of \u200b\u200bthe valve increases, the amount of liquid entering the disc chamber increases, and when the valve is fully opened, its level in both halves is the same. The change in the transmitted torque and fan speed is shown by curves A 2 (see Fig. 80).
In this case, the maximum rotation speed of the veptilator is 3200 min-1, and the power consumption increases to 3.8 kW. The maximum valve opening corresponds to an ambient temperature of 65 ° C. The described engine cooling control can reduce fuel consumption in passenger cars by 1 l / 100 km.
Powerful motors have even more advanced cooling control systems. In Tatra diesel engines, the fan is driven through a fluid coupling, the volume of oil in which is controlled by a thermostat depending on the temperatures of the exhaust gases and the ambient air. The readings of the temperature sensor in the exhaust pipe depend mainly on the engine load and, to a lesser extent, on its speed. The delay of this sensor is very small, so the cooling regulation with it is more perfect.
Cooling control by fan speed is relatively easy in any type of internal combustion engine; this reduces the overall noise generated by the vehicle.
When the engine is located in front of the vehicle, the mechanical drive of the fan causes some difficulties and therefore the electric fan drive is more often used. In this case, the cooling control is greatly simplified. The fan with an electric drive should not have a large power consumption, therefore, they tend to use the effect of cooling by the high-speed air pressure when the car is moving, since with an increase in the engine load, the speed of a car and, therefore, the speed pressure of the air flowing around it grows. The electric fan drive works only for a short time when overcoming long climbs or at high ambient temperatures. The cooling air flow through the fan is regulated by turning on the electric motor using a thermostat,
If the radiator is located far from the engine, such as in a rear-engine bus, the fan is usually hydrostatically driven. A hydraulic pump driven by the bus engine supplies oil under pressure by a hydraulic piston motor with swash plate. Such a drive is more complicated and its use is advisable in high-power engines.
ANDUSE OF HEAT CARRIED OUT WITH EXHAUST GASES
Engine exhaust gases contain a significant amount of thermal energy. It can be used, for example, to heat a car. Heating the air by exhaust gases in the gas-air heat exchanger of the heating system is dangerous due to the possibility of burning out or leakage of its pipes. Therefore, oil or other antifreeze liquid heated by the exhaust gases is used to transfer heat.
It is even more expedient to use the exhaust gases to drive the cooling fan. At high engine loads, the exhaust gases are at the highest temperature, and the engine needs intensive cooling. Therefore, the use of an exhaust gas turbine to drive a cooling system fan is highly advisable and is currently beginning to find application. Such a drive can automatically regulate the cooling, although it is quite expensive.
Ejection cooling can be considered more acceptable in terms of cost. The exhaust gases are sucked off from the ejector cooling air, which is mixed with them and discharged into the atmosphere. Such a device is cheap and reliable since it has no moving parts. An example of an ejection cooling system is shown in Fig. 82.
Ejection cooling has been successfully used in Tatra race cars and in some specialized vehicles. The disadvantage of the system is the high noise level, since the exhaust gases must be supplied directly to the ejector, and the location of the silencer behind it causes difficulties.
The main way to use the energy of exhaust gases is their expansion in a turbine, which is most often used to drive a centrifugal compressor for engine boost. It can also be used for other purposes, for example, for the mentioned fan drive; in turbo compound engines, it is directly connected to the engine crankshaft.
In engines that use hydrogen as fuel, the heat from the exhaust gases, as well as the heat rejected into the cooling system, can be used to heat the hydrides, thereby extracting the hydrogen contained in them. With this method, this heat is accumulated in hydrides, and with new filling of hydride tanks with hydrogen, it can be used for various purposes for heating water, heating buildings, etc.
The energy of the exhaust gases is partially used to improve engine boost, using the resulting fluctuations in their pressure in the exhaust pipeline. The use of pressure fluctuations consists in the fact that after opening the valve, a pressure shock wave arises in the pipeline, passing at the speed of sound to the open end of the pipeline, reflected from it and returning to the valve in the form of a rarefaction wave. During the open state of the valve, the wave can pass through the pipeline several times. At the same time, it is important that by the closing phase of the exhaust valve, a vacuum wave comes to it, which helps to clean the cylinder from exhaust gases and purge it with fresh air. Each branch of the pipeline creates obstacles in the path of pressure waves, therefore the most profitable terms use of pressure fluctuations are created in the case of individual pipelines from each cylinder, having equal lengths from the cylinder head to integration into a common pipeline.
The speed of sound does not depend on the engine speed, therefore, favorable and unfavorable operating conditions in terms of filling and cleaning the cylinders alternate in its entire range. On the curves of the engine power Ne and its average effective pressure pe, this manifests itself in the form of “humps”, which is clearly seen in Fig. 83, which shows the external speed characteristic of a Porsche race car engine. Pressure fluctuations are also used in the intake manifold: the arrival of a pressure wave to the intake valve, especially in the phase of its closing, contributes to the purging and cleaning of the combustion chamber.
If several engine cylinders are connected to a common exhaust pipe, then their number should be no more than three, and the alternation of work should be uniform so that the exhaust gas outlet from one cylinder does not block and does not affect the exhaust process from another. In an in-line four-cylinder engine, the two outer cylinders are usually combined into one common branch, and the two middle cylinders into another. In a six-cylinder in-line engine, these branches are formed by three front and three rear cylinders, respectively. Each of the branches has an independent entrance to the muffler, or at some distance from it the branches are combined and their common input into the muffler is organized.
TURBOCHARGING ENGINE
Turbocharged energy from the exhaust gas is used in a turbine that drives a centrifugal compressor to supply air to the engine. The large mass of air entering the engine under pressure from the compressor helps to increase the specific power of the engine and reduce its specific fuel consumption. The two-stage air compression and expansion of the exhaust gases, carried out in a turbocharged engine, provide a high indicator efficiency of the engine.
If a mechanically driven compressor is used for supercharging, then due to the supply of more air, only the engine power increases. If the expansion stroke is maintained only in the engine cylinders, the exhaust gases leave it under high pressure, and if they are not used in the future, this causes an increase in the specific fuel consumption.
The degree of boost depends on the purpose of the engine. At higher boost pressures, the air in the compressor gets very hot and needs to be cooled at the engine inlet. Currently, turbocharging is used mainly in diesel engines, the increase in power of which by 25-30% does not require a large boost according to the boost pressure, and the engine cooling does not cause any difficulties. This method of increasing diesel power is used most often.
An increase in the amount of air entering the engine makes it possible to operate on lean mixtures, which reduces the output of CO and CHx. Since the power of diesel engines is regulated by the fuel supply, and the supplied air is not throttled, very lean mixtures are used at partial loads, which helps to reduce the specific fuel consumption. Lean ignition in supercharged diesel engines is straightforward as it occurs at high air temperatures. Purging the combustion chamber with supplied air in diesel engines is permissible, since, unlike a gasoline engine, they do not carry fuel into the exhaust pipe.
In a supercharged diesel engine, the compression ratio is usually slightly reduced in order to limit the maximum pressure in the cylinder. Higher air pressures and temperatures at the end of the compression stroke reduce the ignition delay and the engine stiffness becomes less.
Turbocharged diesels have certain problems when it is necessary to quickly increase the engine power. When you press the control pedal, the increase in air supply due to the inertia of the turbocharger lags behind the increase in fuel supply, therefore, at first, the engine runs on a rich mixture with increased smokiness, and only after a certain period of time the mixture composition reaches the required value. The duration of this period depends on the moment of inertia of the turbocharger rotor. An attempt to reduce the rotor inertia to a minimum by reducing the diameter of the turbine and compressor impellers entails the need to increase the turbocharger speed to 100,000 min. Such turbochargers are small and lightweight, an example of one of them is shown in Fig. 84. To obtain high revs of a turbocharger, centripetal turbines are used. Heat transfer from the turbine casing to the compressor casing should be minimized, therefore both casings are well insulated from each other. Depending on the number of cylinders and the connection scheme of their exhaust pipelines, turbines have one or two inlets for exhaust gases. The supercharged diesel engine, thanks to the utilization of the energy of the exhaust gases, makes it possible to achieve very low specific fuel consumption. Recall that the thermal balances of internal combustion engines are given in table. 1 and 2.
For passenger cars, the disadvantage of a diesel engine is its large mass. Therefore, the new diesel engines being created for passenger cars are based mainly on high-speed gasoline engines, since the use of high rotational speeds makes it possible to reduce the diesel mass to an acceptable value.
The fuel consumption of a diesel engine, especially when driving in the city at partial loads, is noticeably lower. Further development of these diesel engines is associated with turbocharging, in which the content of harmful carbon-containing components in the exhaust gases is reduced, and its operation becomes softer. The increase in NOx due to higher combustion temperatures can be reduced by exhaust gas recirculation. The cost of a diesel engine is higher than that of a gasoline engine, however, if there is a lack of oil, its use is more profitable, since it can be from oil! More diesel is caught than high-octane gasoline
Turbocharging of gasoline engines has some peculiarities The temperature of the exhaust raws of gasoline engines is higher, this places higher demands on the material of the turbine blades, but it is not a factor limiting the use of supercharging. It is necessary to regulate the air flow hood, which is especially important at high splice frequencies when the compressor is delivering a large amount of air. Unlike a diesel engine, where the power is controlled by reducing the fuel supply, a similar method is not applicable in a gasoline engine, since the composition of the mixture would be so poor in these modes that ignition would not be guaranteed. Therefore, the air supply at the maximum speed of the turbocharger must be limited. There are several ways to do this. The most commonly used bypass of exhaust gases through a special channel past the turbine, thereby reducing the rotational speed of the turbocharger and the amount of air supplied to it. The scheme of such regulation is shown in Fig. 85.
Exhaust gases from the engine go to the exhaust pipe 10, and then through the turbine 11 in the exhaust silencer 12. At maximum load and high engine speed, the pressure in inlet port 7, transmitted through port 15, opens the bypass valve 13, through which the exhaust gases through the pipeline 14 go directly to the muffler, bypassing the turbine. Less exhaust gas flows into the turbine and air is supplied by the compressor 4 into the inlet 6 decreases 6-8 times. (The design of the EGR valve is shown in figure 86.)
The considered method for regulating the air supply has the disadvantage that the decrease in engine power when the engine control pedal is released does not occur instantaneously and lasts, moreover, longer than the turbine speed drops. When the pedal is depressed again, the required power is reached with a delay, the speed of the turbocharger increases slowly even after the bypass is closed. Such a delay is undesirable during heavy traffic, when you need to quickly brake and then quickly accelerate the car. Therefore, a different control method is used, namely, they additionally use air bypass through the compressor bypass channel 4.
Air enters the engine through air filter 1, mixture ratio regulator 2 firm "Bosch" (Germany) type "K-Jetronic", which controls the fuel injectors 9 (see Ch. 13), then into the inlet pipeline 5, and then the compressor 4 is pumped into inlets and pipes 6 -five. When you quickly release the control pedal, the compressor still rotates, and to reduce the pressure in the channel 6 bypass valve 5 with vacuum in the intake manifold 8 opens and pressurized air from the channel 6 through the same valve 5 is bypassed again into the pipeline 3 in front of the compressor. The pressure equalization is very fast and the turbocharger speed does not drop sharply. The next time you press the pedal, the bypass valve 5 closes quickly and the compressor delivers pressurized air to the engine with a slight delay. This method allows the engine to reach full power within a fraction of a second after pressing the foot control.
A good example of a supercharged gasoline engine is the Porsche 911 (FRG). Initially, it was a naturally aspirated six-cylinder air-cooled engine with a displacement of 2000 cm3, which had a power of 96 kW. In the supercharged version, its working volume was increased to 3000 cm3, and the power was increased to 220 kW, while meeting the requirements for the noise level and the presence of harmful substances in the exhaust gases. The dimensions of the engine have not increased. During the development of the 911 engine, the great experience gained during the creation of the twelve-cylinder racing engine model 917 was used, which already in 1978 developed a power of 810 kW at a speed of 7800 min-1 and a boost pressure of 140 kPa. The engine was equipped with two turbochargers, its maximum torque was 1100 Nm, and its weight was 285 kg. At the nominal engine power mode, the air supply by pipe compressors at a rotational speed of 90,000 min-1 was 0.55 kg / s at an air temperature of 150–160 ° C. At maximum engine power, the exhaust gas temperature reached 1000-1100 ° C. The acceleration of the racing car from standstill to 100 km / h with this engine lasted 2.3 seconds. When creating this racing engine, a perfect turbocharging control system was developed, which allowed to achieve good dynamic qualities of the car. The same control scheme was applied to the Porsche 911 engine.
When fully opened throttle maximum boost pressure in the Porsche 911 engine bypass valve 13 (see fig. 85) is limited to 80 kPa. This pressure is reached already at a speed of 3000 rpm, in the engine speed range of 3000-5500 rpm, the boost pressure is constant and the air temperature behind the compressor is 125 ° C. At maximum engine power, the purge rate reaches 22% of the exhaust gas consumption. The safety valve installed in the intake port is adjusted to a pressure of 110-140 kPa, and in the event of an exhaust gas bypass valve failure, it cuts off the fuel supply, thereby limiting the uncontrolled increase in engine power. At maximum engine power, the air supply from the compressor is 0.24 kg / s. The compression ratio, equal to e \u003d 8.5 in a naturally aspirated engine, was reduced to 6.5 with the introduction of supercharging. In addition, sodium-cooled exhaust valves have been introduced, valve timing has been changed and the cooling system has been improved. At maximum engine power, the turbocharger rotational speed is 90,000 rpm, while the turbine power reaches 26 kW. Cars intended for export to the USA must meet the requirements for the content of harmful substances in the exhaust gases, and therefore the Porsche 911 cars supplied to the USA are additionally equipped with two thermal reactors, a system for supplying secondary air and exhaust gases for afterburning them, as well as exhaust gas recirculation system. The engine power of the Porsche 911 is reduced to 195 kW.
In some other turbocharging control systems, for example ARSthe Swedish company SAAB, electronics are used to regulate the boost pressure. The boost pressure is limited by a valve that regulates the flow of exhaust gases through the wastegate past the turbine. The valve opens when a vacuum occurs in the intake manifold, the value of which is regulated by throttling the air flow between the intake manifold and the compressor inlet.
The throttle valve that regulates the vacuum in the bypass valve is electrically controlled by an electronic device according to signals from the boost pressure, detonation and speed sensors. The knock sensor is a sensitive piezoelectric element installed in the cylinder block and detects the occurrence of knocking knocks. The signal from this sensor limits the vacuum in the control chamber of the bypass valve.
Such a turbocharging control system allows to provide good dynamic qualities of the car, which are necessary, for example, for fast overtaking in heavy traffic conditions. This can be done quickly by switching the engine to maximum boost pressure, since knocking in a relatively cold, part-load engine does not occur instantly. After a few seconds, when the temperatures rise and knock begins to appear, the control device will reduce the boost pressure at the signal from the knock sensor.
The advantage of this regulation is that it allows the use of fuels with different octane numbers in the engine without any changes. When using fuel with an octane rating of 91, a SAAB engine with such a control system can operate for a long time with a boost pressure of up to 70 kPa. The compression ratio of this engine, in which the Bosch K-Jetronic gasoline injection equipment is used, is e \u003d 8.5. The advances made in reducing the fuel consumption of passenger cars through the use of turbocharging have promoted its use in motorcycle construction. Here we should mention the Japanese firm "Honda", which was the first to use turbocharging in a two-cylinder liquid-cooled engine of the model "SH500 ”to increase its power and reduce fuel consumption. The use of turbochargers in engines with a small displacement has a number of difficulties associated with the need to obtain the same boost pressures as in high-power engines, but at low air consumption. The boost pressure depends mainly on the circumferential speed of the compressor wheel, and the diameter of this wheel is determined by the required air supply. It is therefore necessary that the turbocharger has a very high rotational speed with small impeller diameters. The diameter of the compressor wheel in the aforementioned 500 cm3 Honda engine is 48.3 mm and at a boost pressure of 0.13 MPa the turbocharger rotor rotates at a frequency of 180,000 min-1. The maximum permissible speed of this turbocharger is 240,000 min-1.
With an increase in boost pressure above 0.13 MPa, the exhaust gas bypass valve (Fig. 87) is opened, controlled by the boost pressure in the chamber, and part of the exhaust gases, bypassing the turbine, is sent to the exhaust pipeline, which limits a further increase in the compressor speed. The bypass valve opens at an engine speed of about 6500 min-1, and with its further increase, the boost pressure no longer increases.
The amount of fuel injected by the injector required to obtain the required mixture composition is determined by a computing device located above the rear wheel of the motorcycle, which also processes information from the temperature sensors of the incoming air and coolant, the throttle position sensor, air pressure sensors, and engine speed sensor.
The main advantage of a supercharged engine is the reduction in fuel consumption while increasing engine power. Motorcycle "Honda SHThe naturally aspirated 500 "consumes 4.8 l / 100 km, while the same supercharged CX 500 7X uses only 4.28 l / 100 km. Weight of the Honda motorcycle SH500 g "is 248 kg, which is more than 50 kg higher than the mass of motorcycles of a similar class with an engine displacement of 500-550 cm3 (for example, a motorcycle" Kawasaki KZ550 ”has a mass of 190 kg). At the same time, however, the dynamic qualities and the maximum speed of the motorcycle "Honda CX 500 7" are the same as those of motorcycles with twice the displacement. At the same time, the braking system has been improved in connection with the increase in the speed qualities of this motorcycle. The Honda CX 500 G engine is designed for even higher speeds and its maximum rotational speed is 9000 min-1.
A decrease in the average fuel consumption is also achieved by the fact that when the motorcycle is moving at an average operating speed, the pressure in the intake manifold is equal to atmospheric or even slightly lower than it, that is, the use of boost is very insignificant. Only when the throttle valve is fully opened and, consequently, the quantity and temperature of the exhaust gases increase, the turbocharger speed and boost pressure increase, and thus the engine power increases. Some lag in the increase in engine power with a sharp opening of the throttle valve occurs and is associated with the time required to accelerate the turbocharger.
General scheme power plant motorcycle "Honda CX 500 T "turbocharged is shown in fig. 87. Large fluctuations in air pressure in the intake manifold of a two-cylinder engine with an uneven order of operation of the cylinders are extinguished by a chamber and a damping receiver. When starting the engine, the valves prevent air backflow caused by a large overlap of the valve timing. The liquid cooling system eliminates the hot air supply to the driver's feet, which occurs with air cooling. The cooling system radiator is blown by an electrically driven fan. The short exhaust pipe to the turbine reduces the energy loss of the exhaust gases and helps to reduce fuel consumption. The maximum speed of the motorcycle is 177 km / h.
BLOWER TYPE "COMPREX"
A very interesting method of boosting Comprex, developed by Brown & Boveri, Switzerland, is to use exhaust gas pressure acting directly on the air flow to the engine. The resulting engine performance is the same as in the case of using a turbocharger, but the turbine and centrifugal compressor, for the manufacture and balancing of which require special materials and high-precision equipment, are absent.
A diagram of the Comprex-type pressurization system is shown in Fig. 88. The main part is a vane rotor rotating in a housing with a rotational speed three times higher than the rotational speed of the engine crankshaft. The rotor is installed in the housing on rolling bearings and is driven by a V-belt or toothed belt. The compressor drive of the "Comprex" type consumes no more than 2% of the engine power. The “Comprex” unit is not a compressor in the full sense of the word, since its rotor has only channels parallel to the axis of rotation. In these channels, the air entering the engine is compressed by the pressure of the exhaust gases. The rotor end clearances ensure the distribution of exhaust gases and air through the rotor channels. Radial plates are located on the outer contour of the rotor, which have small gaps with the inner surface of the housing, due to which channels are formed that are closed on both sides by end caps.
There are windows in the right cover for supplying exhaust gases from the engine to the unit housing and r -for the removal of exhaust gases from the housing into the exhaust pipe and then into the atmosphere There are windows in the left cover bto serve compressed air in the engine and windows dfor supplying fresh air to the housing from the intake manifold e.The movement of the channels during the rotation of the rotor causes them to alternately connect to the exhaust and intake pipes of the engine.
When opening a window anda pressure shock wave arises, which moves at the speed of sound to the other end of the exhaust pipeline and simultaneously directs the exhaust gases into the rotor channel without mixing them with air. When this pressure wave reaches the other end of the exhaust pipeline, window b will open and the air compressed by the exhaust gases in the rotor channel will be pushed out of it into the pipeline into the engine. However, even before the exhaust gases in this rotor channel approach its left end, the window closes first. andand then the window b, and this channel of the rotor with the exhaust gases under pressure in it will be closed on both sides by the end walls of the housing.
With further rotation of the rotor, this channel with the exhaust gases will come to the window rin the exhaust pipe, the wire and exhaust gases will come out of the channel into it. When the channel moves past the windows rthe outgoing exhaust gases are ejected through the windows dfresh air, which, filling the entire channel, blows and cools the rotor. Going through the windows rand d,the rotor channel, filled with fresh air, is again closed on both sides by the end walls of the housing and is thus ready for the next cycle. The described cycle is very simplified in comparison with what happens in reality and is carried out only in a narrow range of engine speed. This is the reason why this supercharging method, which has been known for 40 years, is not used in cars. Over the past 10 years, by the work of Brown & Bover, the Comprex supercharging has been significantly improved, in particular, an additional chamber has been introduced in the end cover, which provides reliable air supply in a wide range of engine speed, including at low speed values.
Comprex supercharging was tested on four-wheel drive vehicles high cross-country ability the Austrian company "Steyer-Daimler-Pooh", which were equipped with diesel engines "Opel Record 2,3D" and "Mercedes-Benz 200D".
The advantage of the Comprex method in comparison with turbocharging is that there is no lag in the increase in boost pressure after pressing the control pedal. The efficiency of the turbocharging system is determined by the energy of the exhaust gases, which depends on their temperature. If, for example, at full engine power, the exhaust gas temperature is 400 ° C, then in winter it takes several minutes to reach it. A significant advantage of the "Comprex" method is also in obtaining a large engine torque at low speeds, which allows the use of a gearbox with fewer steps.
A quick build-up of engine power when the foot switch is depressed is especially desirable for racing cars The Italian firm Ferrari is testing the Comprex pressurization method in its racing cars, because when using turbocharging, the engine must react quickly to the pedal position when cornering in a racing car. application of the previously described complex control system.
When testing the Comprex pressurization system on six-cylinder engines of Ferrari racing cars F1there was a very fast reaction of the engine to movement of the control pedal
To obtain the maximum boost pressure on these engines, charge air cooling is used. More air flows through the rotor of the Comprex unit than is required by the engine, since part of the air is used to cool the boost unit. This is very beneficial for racing engines, which even at the start run at almost full air flow through the intercooler. Under these conditions, the engine with the Comprex unit will be in the best temperature condition by the time of start to reach full power.
The use of a Comprex pressurization unit instead of a turbocharger reduces engine noise, since it operates at a lower speed. In the early stages of development, the rotor speed was the cause of the noise of the same frequency as that of the turbocharger. This drawback was eliminated by the uneven pitch of the channels around the rotor circumference.
When using the "Comprex" system, exhaust gas recirculation is greatly simplified, which is used to reduce the content in them NOx.Typically, recirculation is carried out by taking part of the exhaust gases from the exhaust pipe, dosing them, cooling them and supplying them to the engine intake manifold. In the "Comprex" system, this scheme can be much simpler, since the mixing of exhaust gases with the fresh air stream and their cooling occurs directly in the rotor channels.
WAYS TO INCREASE THE MECHANICAL EFFICIENCY OF THE INTERNAL COMBUSTION ENGINE
Mechanical efficiency reflects the relationship between the indicated and effective engine power. The difference in these values \u200b\u200bis caused by losses associated with the transfer of gas forces from the piston crown to the flywheel and with the drive of the engine auxiliary equipment. All these losses must be known exactly when the task is to improve the fuel efficiency of the engine.
Most of the losses are caused by friction in the cylinder, less by friction in well-lubricated bearings and the drive equipment necessary for the engine to operate. Losses due to air entering the engine (pumping losses) are very important as they increase in proportion to the square of the engine speed.
The power losses required to drive the equipment that ensure the operation of the engine include the power to drive the gas distribution mechanism, oil, water and fuel pumps, and the cooling fan. With air cooling, the air supply fan is an integral part of the engine when it is tested on the bench, while liquid-cooled engines often do not have a fan and a radiator during tests, and water from an external cooling circuit is used for cooling. If the power consumption of the fan of a liquid-cooled engine is not taken into account, then this gives a noticeable overestimation of its economic and power indicators in comparison with an air-cooled engine.
Other losses for the drive of equipment are associated with the generator, pneumatic compressor, hydraulic pumps necessary for lighting, ensuring the operation of instruments, braking system, and steering a car. When testing an engine on a brake stand, it should be precisely defined what is considered an accessory and how to load it, as this is necessary for an objective comparison of the characteristics of different engines. In particular, this applies to the oil cooling system, which, when the vehicle is moving, is cooled by blowing the oil pan with air that was absent during tests on a brake stand. When testing an engine without a fan on the bench, the conditions for blowing air through the pipelines are not reproduced, which causes an increase in temperatures in the intake pipe and leads to a decrease in the filling factor and engine power.
Accommodation air filter and the value of the resistance of the exhaust pipe should be consistent with the operating conditions of the engine in the car. These important features must be considered when comparing characteristics various engines or one engine designed for use in different conditions, for example, in a passenger car or truck, tractor or to drive a stationary generator, compressor, etc.
With a decrease in the engine load, its mechanical efficiency deteriorates, since the absolute value of most of the losses does not depend on the load. An illustrative example is the operation of the engine without load, that is, at idle speed, when the mechanical efficiency is zero and all the indicated engine power is spent on overcoming its losses. When the engine is loaded by 50% or less, the specific fuel consumption compared to full load increases significantly, and therefore it is completely uneconomical to use an engine that has more than required power for the drive.
The mechanical efficiency of the engine depends on the type of oil used. Application in winter time oils with higher viscosity leads to an increase in fuel consumption. Engine power at high altitudes above sea level decreases due to a decrease in atmospheric pressure, but its losses practically do not change, as a result of which the specific fuel consumption increases in the same way as it occurs with a partial engine load.
FRICTION LOSSES IN CYLINDER PISTON GROUP AND BEARINGS
The greatest losses in the engine are caused by the friction of the piston in the cylinder. The lubrication conditions for the cylinder walls are far from being satisfactory. The oil layer on the cylinder wall with the piston in BDC is exposed to hot exhaust gases. To reduce oil consumption, the oil scraper ring removes part of it from the cylinder wall when the piston moves to BDC, however, a layer of lubricant between the piston skirt and the cylinder remains.
The first compression ring causes the greatest friction. When the piston moves to TDC, this ring rests on the lower surface of the piston groove of the piston and the pressure arising from the compression and then combustion of the working mixture presses it against the cylinder wall. Since the lubrication regime of the piston ring is the least favorable due to the presence of dry friction and high temperature, the friction losses are the highest here. The lubrication regime for the second compression ring is more favorable, but the friction remains significant. Therefore, the number of piston rings also affects the amount of friction loss in the cylinder-piston group.
Another unfavorable factor is the pressing of the piston near the TDC against the cylinder wall by the gas pressure and the inertial forces of the reciprocating masses. In high-speed automobile engines, inertial forces are greater than those of gas. Therefore, the connecting rod bearings have the greatest load at the TDC of the exhaust stroke, when the connecting rod is stretched by inertial forces applied to its upper and lower heads.
The force acting along the connecting rod is decomposed into forces directed along the axis of the cylinder and normal to its wall.
Rolling bearings in an engine are advantageous to use when there are large forces acting on them. It is advisable, for example, to place "the rocker arms of the valves on needle bearings. Roller bearings were also previously used as piston pin bearings in the connecting rod, especially in high-power two-stroke engines. The piston and piston pin bearing of a two-stroke engine in most cases are loaded in only one direction, therefore the required oil film cannot form in the sleeve bearing.For good lubrication of the sleeve bearing, in the upper head of the connecting rod, along the entire length of its sleeve, in this case, transverse lubrication grooves are made, located at such a distance from each other that an oil film could form in this place during oscillation ...
To obtain low frictional losses in the cylinder-piston group, it is necessary to have pistons with a small mass, a small number of piston rings and a protective layer on the piston skirt that protects the piston from scuffing and seizure.
LOSSES DURING GAS EXCHANGE
To fill the cylinder with air, it is necessary to create a pressure difference between the cylinder and the external environment. The vacuum in the cylinder at the intake, acting in the direction opposite to the piston movement, and braking the rotation of the crankshaft, depends on the valve timing, the diameter of the intake manifold, as well as the shape of the intake channel, which is necessary, for example, to create air rotation in the cylinder. The engine in this part of the cycle acts as an air pump and a part of the indicated engine power is consumed to drive it.
For a good filling of the cylinder, it is necessary that the pressure loss, proportional to the square of the engine speed, during filling is the smallest. Friction losses in the cylinder-piston group have a similar nature of dependence on the rotational speed, and since this type of losses prevails among others, the total losses also depend on the second degree of the engine speed. Therefore, the mechanical efficiency decreases with increasing rotational speed, and the specific fuel consumption deteriorates.
At maximum engine power, the mechanical efficiency is typically 0.75, and as the engine speed is further increased, the effective power drops rapidly. At maximum engine speed and partial loads, the effective efficiency is minimal.
Losses during gas exchange also include the energy costs associated with blowing the crankcase of the crankshaft. Single-cylinder four-stroke engines have the greatest losses, in which air is sucked into the crankcase at each stroke of the piston and is pushed out of it again. A large volume of air pumped through the crankcase also has two-cylinder engines with V-shaped and opposed cylinder arrangements. This type of loss can be reduced by installing a check valve that creates a vacuum in the crankcase. The crankcase vacuum also reduces oil losses due to leaks. In multi-cylinder engines, in which one piston moves down and the other up, the volume of gas in the crankcase does not change, but the adjacent sections of the cylinders must communicate well with each other.
LOSSES ON THE DRIVE OF AUXILIARY EQUIPMENT OF THE ENGINE
Equipment drive losses are often underestimated, although they have a large impact on the mechanical efficiency of a motor. The losses on the drive of the gas distribution mechanism have been well studied. The work expended in opening the valve is partially recovered when the valve spring closes the valve and thus drives the camshaft. Losses for the gas distribution drive are relatively small and with their reduction it is possible to obtain only a small saving in power costs for the drives. Sometimes the camshaft is mounted on antifriction bearings, but this is only used on race car engines.
More attention should be paid to the oil pump. If the dimensions of the pump and the oil flow through it are overestimated, then most of the oil is discharged through the pressure reducing valve at high pressure, there are significant losses for the oil pump drive. At the same time, it is necessary to have reserves in the lubrication system in order to provide sufficient pressure for lubricating plain bearings, including worn ones. In this case, a low oil supply by the pump leads to a decrease in pressure at low engine speeds and during prolonged operation at full load. The pressure reducing valve must be closed under these conditions and the entire oil supply must be used for lubrication. Low power is consumed to drive the fuel pump and ignition distributor. Also, the alternator consumes little energy. A significant part of the effective power, namely 5-10%, is spent on driving the fan and pump of the cooling system, which are required to remove heat from the engine. This has already been discussed. There are, as can be seen, several ways to improve the mechanical efficiency of a motor.
A small amount of energy can be saved on driving the fuel pump and opening the injectors. To a somewhat greater extent this is possible in diesel engines.
LOSSES ON THE DRIVE OF ADDITIONAL EQUIPMENT OF THE VEHICLE
A car is also usually equipped with equipment that consumes some of the effective engine power, and thereby reduces the rest of it going to drive the car. In a passenger car, such equipment is used in a limited number, mainly various amplifiers used to facilitate driving a car, for example, steering, clutch drive, brake drive. The air conditioning of the car also requires a certain amount of energy, especially for the air conditioning. Energy is also needed for various hydraulic drives, such as moving seats, opening windows, roofs, etc.
There is much more additional equipment in a truck. Typically, a braking system using a separate energy source, dump bodies, self-loading devices, a device for raising spare wheels, etc. are used. special purpose such mechanisms are even more widely used. These cases of energy consumption must also be taken into account in the total fuel consumption.
The most important of these devices is a compressor for creating constant air pressure in a pneumatic brake system The compressor works constantly, filling the air receiver, part of the air from which is released into the atmosphere through the pressure reducing valve without further use. For hydraulic systems high pressureservicing additional equipment are characterized mainly by losses in pressure reducing valves. They usually use a valve, which, after reaching the working pressure in the accumulator, turns off the further supply of working fluid to it and controls the bypass line between the pump and the tank.
COMPARISON OF MECHANICAL LOSSES IN PETROL AND DIESEL ENGINES
Comparative data on mechanical losses measured under the same operating conditions of a gasoline engine with a compression ratio of e \u003d 6 and a diesel engine with a compression ratio of e \u003d 16 (Table 11, A).
For a gasoline engine, in addition, in table. 11, B also compares mechanical losses at full and partial loads.
Table 11.A. Average pressure of various types of mechanical losses in gasoline and diesel engines (1600 min-1), MPa
| Loss type | engine's type | |
| Gasolinee \u003d 6 | Diesel \u003d 16 | |
| 0,025 | 0,025 | |
| Drive of water, oil and fuel pumps | 0,0072 | 0,0108 |
| Gas distribution mechanism drive | 0,0108 | 0,0108 |
| Losses in main and brass bearings | 0,029 | 0,043 |
| 0,057 | 0,09 | |
| Mechanical losses, total | 0,129 | 0,18 |
| Average effective pressure | 0,933 | 0,846 |
| Mechanical efficiency,% | 87,8 | 82,5 |
Table 11.B. Average pressure of various types of mechanical losses in a gasoline engine (1600 min-1, e \u003d 6) at various loads, MPa
| Loss type | ||
| 100 % | 30 % | |
| Pumping losses (losses for gas exchange) | 0,025 | 0,043 |
| Timing mechanism and auxiliary equipment drive | 0,0179 |
0,0179 |
| Losses in the crank mechanism | 0,0287 | 0,0251 |
| Losses in the cylinder-piston group | 0,0574 | 0,05 |
| Mechanical losses, total | 0,129 | 0,136 |
| Average effective pressure | 0,933 | 0,280 |
| Mechanical efficiency,% | 87,8 | 67,3 |
The total losses, as can be seen from the table. 11 are relatively small because they were measured at low rpm (1600 rpm). With an increase in the rotational speed, losses increase due to the action of the forces of inertia of the translationally moving masses, which increase in proportion to the second power of the rotational speed, as well as the relative speed in the bearing, since the viscous friction is also proportional to the square of the speed. It is also interesting to compare the indicator diagrams in the cylinders of the two engines under consideration (Fig. 89). The pressure in the cylinder of a diesel engine is slightly higher than that of a gasoline engine, and its duration is longer. Thus, the gases press the rings against the cylinder wall with greater force and for a longer time, therefore, the friction losses in the cylinder-piston group are greater in the diesel engine. The larger dimensions compared to a gasoline engine, especially the diameter of the bearings in a diesel engine, also contribute to an increase in mechanical losses.
Bearing friction is caused by shear stresses in the oil film. It linearly depends on the size of the friction surfaces and is proportional to the square of the shear rate. The oil viscosity and, to a lesser extent, the oil film thickness in the bearings have a significant effect on friction. Gas pressure in the cylinder has almost no effect on bearing losses.
INFLUENCE OF CYLINDER DIAMETER AND PISTON STROKE ON EFFICIENT EFFICIENCY OF INTERNAL COMBUSTION ENGINE
Earlier, it was about reducing to a minimum heat loss to increase the indicator efficiency of the engine, and it was mainly about reducing the ratio of the surface of the combustion chamber to its volume. The volume of the combustion chamber is to some extent an indication of the amount of heat input. The calorific value of the incoming charge in a gasoline engine is determined by the ratio of air to fuel, close to stoichiometric. Clean air is supplied to the diesel engine, and the fuel supply is limited by the degree of incompleteness of combustion, at which smoke appears in the exhaust gases.Therefore, the relationship between the amount of introduced heat and the volume of the combustion chamber is quite obvious
The sphere has the smallest surface-to-volume ratio. The heat is removed to the surrounding space by the surface, so the ball-shaped mass is cooled to the least degree. These obvious relationships are taken into account when designing the combustion chamber. However, it should be borne in mind the geometric similarity of engine parts of different sizes. As you know, the volume of a sphere is 4 / 3lR3, and its surface is 4lR2, and, thus, the volume with increasing diameter increases faster than the surface, and, therefore, a sphere with a larger diameter will have a smaller surface-to-volume ratio. If the surfaces of a sphere of different diameters have the same temperature differences and the same heat transfer coefficients a, then the large sphere will cool more slowly.
Motors are geometrically similar when they have the same design but differ in size. If the first engine has a cylinder diameter, for example, equal to one, and the second engine has he is at 2times more, then all linear dimensions of the second engine will be 2 times, surfaces - 4 times, and volumes - 8 times larger than those of the first engine. However, it is not possible to achieve complete geometric similarity, since the dimensions of, for example, spark plugs and fuel injectors are the same for engines with different cylinder bore sizes.
From the geometric similarity, it can be concluded that a larger cylinder also has a more acceptable surface-to-volume ratio, therefore, its heat losses when the surface is cooled under the same conditions will be less.
When determining the power, however, some limiting factors must be considered. Engine power depends not only on the size, i.e., the volume of the engine cylinders, but also on the engine speed, as well as the average effective pressure. The engine speed is limited by the maximum average piston speed, weight and design perfection of the crank mechanism. The maximum average piston speeds of gasoline engines are in the range of 10-22 m / s. For passenger car engines, the maximum average piston speed reaches 15 m / s, and the average effective pressure at full load is close to 1 MPa.
Engine displacement and dimensions are not only determined by geometric factors. For example, the thickness of the walls is determined by the technology, not by the load on them. Heat transfer through the walls does not depend on their thickness, but on the thermal conductivity of their material, heat transfer coefficients on the wall surfaces, temperature differences, etc. Oscillations of gas pressure in pipelines propagate at the speed of sound regardless of the engine size, bearing clearances are determined by the properties of the oil film and etc. Some conclusions regarding the influence of the geometric dimensions of the cylinders, however, must be made.
ADVANTAGES AND DISADVANTAGES OF A CYLINDER WITH A LARGE WORKING VOLUME
A cylinder with a larger working volume has a lower relative heat loss to the walls. This is well confirmed by the examples of stationary diesel engines with large displacement cylinders, which have very low specific fuel consumption. In the case of passenger cars, however, this is not always the case.
An analysis of the engine power equation shows that the highest engine power can be achieved with a small piston stroke.
The average piston speed can be calculated as
where: S is the piston stroke, m; n - rotation frequency, min-1.
When the average speed of the piston C p is limited, the rotation frequency can be the higher, the smaller the piston stroke. The power equation for a four-stroke engine is
where: Vh - engine volume, dm3; n - rotation frequency, min-1; pe - average pressure, MPa.
Consequently, engine power is directly proportional to its speed and displacement. Thus, at the same time, the opposite requirements are imposed on the engine - a large cylinder displacement and a short stroke. The compromise solution is to use more cylinders.
The most preferred displacement for one cylinder of a high speed gasoline engine is 300-500 cc. An engine with a small number of such cylinders is poorly balanced, and with a large number of such cylinders it has significant mechanical losses and therefore has an increased specific fuel consumption. An eight-cylinder engine with a displacement of 3000 cm3 has a lower specific fuel consumption than a twelve-cylinder with the same displacement.
To achieve low fuel consumption, it is advisable to use engines with a small number of cylinders. However, a single-cylinder engine with a large displacement is not used in cars, since its relative mass is large, and balancing is possible only with the use of special mechanisms, which leads to an additional increase in its mass, size and cost. In addition, the large torque unevenness of a single-cylinder engine is unacceptable for vehicle transmissions.
The smallest number of cylinders in a modern automobile engine is two. Such engines are successfully used in especially small cars (Citroen 2 CV, Fiat 126). From the point of view of balance, the next in a series of expedient applications is a four-cylinder engine, however, three-cylinder engines with a small working volume of cylinders are now beginning to be used, since they allow you to obtain low fuel consumption. In addition, fewer cylinders simplify and reduce the cost of engine accessories, as the number of spark plugs, injectors, and plunger pairs of the high pressure fuel pump is reduced. When positioned transversely in a car, such an engine has a shorter length and does not restrict the rotation of the steered wheels.
The three-cylinder engine allows the use of basic parts unified with the four-cylinder engine: a cylinder liner, a piston set, a connecting rod set, a valve mechanism. The same solution is possible for a five-cylinder engine, which makes it possible, if necessary to increase the power range upwards from the base four-cylinder engine, to avoid the transition to a longer six-cylinder engine.
The advantages of using diesel engines with a large cylinder displacement have already been pointed out. In addition to reducing the heat loss during combustion, this makes it possible to obtain a more compact combustion chamber, in which, at moderate compression ratios, higher temperatures are created at the time of fuel injection. For a cylinder with a large displacement, nozzles with a large number nozzle holes that are less sensitive to carbon formation.
RATIO OF PISTON STROKE TO CYLINDER DIAMETER
The quotient of dividing the size of the piston stroke S by the size of the cylinder diameter Dis the commonly used S / D ratio . The point of view on the magnitude of the piston stroke has changed during the development of engine building.
At the initial stage of automobile engine building, the so-called tax formula was in force, on the basis of which the levied tax on engine power was calculated taking into account the number and diameter D its cylinders. The classification of engines was also carried out in accordance with this formula. Therefore, preference was given to engines with a large piston stroke in order to increase engine power within this tax category. Engine power increased, but the increase in speed was limited by the permissible average piston speed. Since the engine's gas distribution mechanism during this period was not designed for high revs, the limitation of the revs by the piston speed did not matter.
As soon as the described tax formula was abolished, and the classification of engines began to be carried out in accordance with the working volume of the cylinder, the piston stroke began to decrease sharply, which made it possible to increase the rotational speed and, thereby, the engine power. Larger cylinders made possible the use of valves large sizes... Therefore, short-stroke engines were created with an S / D ratio reaching 0.5. Improvement of the gas distribution mechanism, especially when using four valves in the cylinder, made it possible to bring the nominal engine speed to 10,000 rpm and more, as a result of which the specific power increased rapidly
At present, much attention is paid to reducing fuel consumption. The studies of the effect of S / D carried out for this purpose have shown that short-stroke engines have an increased specific fuel consumption. This is caused by the large surface of the combustion chamber, as well as a decrease in the mechanical efficiency of the engine due to the relatively large value of the translationally moving masses of the parts of the connecting rod-piston set and the increase in losses on the drives of auxiliary equipment.With a very short stroke, the connecting rod must be lengthened in order to bottom part the piston skirts are not touched by the crankshaft counterweights. With a decrease in its stroke, the mass of the piston has slightly decreased and with the use of recesses and notches on the piston skirt.To reduce the emission of toxic substances in the exhaust gases, it is more expedient to use engines with a compact combustion chamber and a longer piston stroke.Therefore, at present, from engines with a very low S / D refuse.
Dependence of the average effective pressure on the S / D ratio the best racing engines, where the decrease in q is clearly visible, at low S / D ratios, is shown in Fig. 90 Currently, an S / D ratio equal to or slightly greater than one is considered more advantageous. Although, with a short piston stroke, the ratio of the cylinder surface to its working volume at the piston position at BDC is less than that of long-stroke engines, the lower cylinder zone is not so important for heat removal, since the gas temperature already drops noticeably
A long-stroke engine has a more favorable ratio of the cooled surface to the volume of the combustion chamber when the piston is at TDC, which is more important, since during this period of the cycle the gas temperature, which determines the heat loss, is the highest. Reducing the heat transfer surface during this phase of the expansion process reduces heat losses and improves the indicated engine efficiency.
OTHER WAYS TO REDUCE ENGINE FUEL CONSUMPTION
The engine operates with minimum fuel consumption only in a certain area of \u200b\u200bits characteristics.
When operating a vehicle, its engine power should always be located on the curve of the minimum specific fuel consumption. In a passenger car, this condition is satisfied if a four- and five-speed gearbox is used, and the fewer the gears, the more difficult it is to fulfill this condition. When driving on a horizontal section of the road, the engine does not work optimally even when the fourth gear is engaged. Therefore, for optimal engine load, the car must be accelerated in the highest gear until the maximum legal speed is reached. Further, it is advisable to shift the gearbox to neutral, turn off the engine and coast to a drop in speed, for example, to 60 km / h, and then turn on the engine and top gear in the box again and, with optimal pressure on the engine control pedal, bring the speed back to 90 km / h.
Such a car driving in the "acceleration-reel" way. This driving method is acceptable for economy competition because the engine is either running in the economy range or off. However, it is not suitable for real operation of a car in heavy traffic.
This example shows one way to reduce fuel consumption. Another way to minimize specific fuel consumption is to limit engine power while maintaining good mechanical efficiency. The negative effect of partial load on mechanical efficiency has already been shown in Table. 11A. In particular, from table. 11B it can be seen that when the engine load decreases from 100% to 30%, the share of mechanical losses in the indicator work increases from 12% to 33%, and the mechanical efficiency drops from 88% to 67%. A power level of 30% of the maximum can be achieved with only two cylinders of a four-cylinder engine running.
SHUT OFF CYLINDERS
If, at partial load of a multi-cylinder engine, several cylinders are turned off, then the rest will operate at a higher load with better efficiency. Thus, when an eight-cylinder engine is operating at part load, the entire volume of air can be directed to only four cylinders, their load will double and the effective efficiency of the engine will increase. The cooling surface of the combustion chambers of four cylinders is less than that of eight, so the amount of heat dissipated by the cooling system is reduced and fuel consumption can be reduced by 25%.
Valve drive control is usually used to shut off the cylinders. If both valves are closed, then the mixture does not enter the cylinder and the gas permanently in it is sequentially compressed and expanded. The work expended in this case for compressing the gas is again released when it expands under conditions of little heat dissipation by the cylinder walls. The mechanical and indicator efficiency in this case is improved in comparison with the efficiency of an eight-cylinder engine operating on all cylinders at the same effective power.
This method of turning off the cylinders is very convenient, since the cylinder is turned off automatically when the engine switches to partial loads and is turned on almost instantly when you press the control pedal. Consequently, the driver can use the full engine power at any time to overtake or quickly climb a hill. Fuel economy is especially evident when driving in the city. Off cylinders have no pumping losses and do not supply air to the exhaust line. When driving downhill, the disengaged cylinders offer less resistance, the engine braking is reduced, and the vehicle coasts a longer distance, as with a freewheel.
It is convenient to turn off the cylinder of an overhead valve engine with a lower camshaft using an electromagnetically movable valve rocker arm stop. When the solenoid is turned off, the valve remains closed, since the rocker arm is rotated by the camshaft cam around the point of contact with the end of the valve stem, and the rocker arm can move freely at the same time.
With an eight-cylinder engine, two or four cylinders are switched off so that the alternation of the working cylinders is as even as possible. In a six-cylinder engine, one to three cylinders are switched off. Testing is also under way to shut off two cylinders of a four-cylinder engine.
Such a shutdown of valves in an engine with an overhead camshaft is difficult, therefore, other methods of shutting down the cylinders are used. For example, half of the cylinders of a six-cylinder bMW engine (FRG) is turned off so that ignition and injection are turned off for three cylinders, and the exhaust gases from three working cylinders are discharged through three disconnected cylinders and can expand further. This process is carried out by valves in the intake and exhaust pipes. The advantage of this method is that the switched off cylinders are constantly heated by the passing exhaust gases.
The Porsche 928 V-8 engine with cylinder deactivation has two almost completely separated four-cylinder V-sections. Each of them is equipped with an independent inlet pipeline, while the gas distribution mechanism does not have a shutdown of the valve drives. One of the engines is shut down by closing the throttle and stopping petrol injection, and tests have shown that pumping losses will be lowest at a small throttle opening. Throttle valves of both sections are equipped with independent drives. The section to be switched off constantly supplies a small amount of air to the common exhaust pipe, which is used for afterburning the exhaust gases in the thermal reactor. This eliminates the need for a dedicated secondary air pump.
When the eight-cylinder engine is divided into two four-cylinder sections, one of them is adjusted for a high torque at a low rpm and is constantly in operation, and the second is set to maximum power and is turned on only when it is necessary to have a power close to maximum. Engine sections can have different valve timing and different intake pipe lengths.
The multi-parameter characteristics of the Porsche 928 engine during operation of eight (solid curves) and four cylinders (dashed curves) are shown in Fig. 91. Areas of improvement in specific fuel consumption by disabling four engine cylinders are shaded. For example, at a rotational speed of 2000 min-1 and a torque of 80 Nm, the specific fuel consumption when all eight cylinders of the engine are operating is 400 g / (kW 350 g / (kWh).
Even more noticeable fuel savings can be obtained at low vehicle speeds. The difference in fuel consumption with uniform movement along the horizontal section of the highway is shown in Fig. 92. For an engine with four disengaged cylinders (dashed curve) at a speed of 40 km / h, fuel consumption drops by 25%: from 8 to 6 l / 100 km.
But fuel economy in the engine can be achieved not only by turning off the cylinders. In the new Porsche engines TOP(“Thermodynamically optimized Porsche engine”) all possible ways to improve the indicator efficiency of a traditional gasoline engine have been implemented. The compression ratio was increased first from 8.5 to 10, and then, by changing the shape of the piston crown, to 12.5, while increasing the intensity of rotation of the charge in the cylinder during the compression stroke. The engines "Porsche 924" and "Porsche 928" upgraded in this way have reduced specific fuel consumption by 6-12%. Applied in this electronic system ignition, setting the optimal ignition timing depending on the engine speed and load, increases the efficiency of the engine when operating at partial loads in conditions of lean mixtures, and also eliminates detonation at maximum loads.
Turning off the engine when the vehicle stops at intersections also saves fuel. When the engine is idling at a speed lower than 1000 rpm, and the coolant temperature is more than 40 ° C, the ignition is switched off after 3.5 s. The engine starts up again only after pressing the control pedal. This reduces fuel consumption by 25-35%, and therefore the Porsche petrol engines TOPin terms of fuel efficiency, they can compete with diesel engines.
Mercedes-Benz has also attempted to reduce fuel consumption in the V-8 by disabling the cylinders. Disconnection was achieved using an electromagnetic device breaking the rigid connection between the cam and the valve. In urban driving conditions, fuel consumption was reduced by 32%.
PLASMA IGNITION
It is possible to use lean mixtures to reduce fuel consumption and the content of harmful substances in the exhaust gases, but spark ignition is difficult. Guaranteed spark ignition occurs when the air / fuel mass ratio is no more than 17. With poorer compositions, misfires occur, which leads to an increase in the content of harmful substances in the exhaust gases.
By creating a stratified charge in the cylinder, it is possible to burn a very lean mixture, provided that a rich mixture is formed in the area of \u200b\u200bthe spark plug. The rich mixture is highly flammable, and the flame, thrown into the volume of the combustion chamber, ignites the lean mixture located there.
IN last years research is underway on the ignition of lean mixtures by plasma and laser methods, in which several foci of combustion are formed in the combustion chamber, since the mixture is ignited simultaneously in different zones of the chamber. As a result, knocking problems are eliminated, and the compression ratio can be increased even when using low-octane fuel. In this case, ignition of lean mixtures with an air / fuel ratio of up to 27 is possible.
During plasma ignition, the electric arc forms a high concentration of electrical energy in an ionized spark gap of a sufficiently large volume. At the same time, temperatures up to 40,000 ° C develop in the arc, that is, conditions are created similar to arc welding.
However, it is not so easy to implement the plasma ignition method in an internal combustion engine. A plasma spark plug is shown in fig. 93. A small chamber is made under the central electrode in the spark plug insulator. When a long electric discharge occurs between the central electrode and the body of the candle, the gas in the chamber heats up to a very high temperature and, expanding, exits through the hole in the body of the candle into the combustion chamber. A plasma torch with a length of about 6 mm is formed, due to which several hot spots of flame arise, contributing to the ignition and combustion of the lean mixture.
Another type of plasma ignition system uses a small high-pressure pump that supplies air to the electrodes when the arc is generated. The volume of ionized air formed during the discharge between the electrodes enters the combustion chamber.
These methods are very complex and do not apply to automobile engines. Therefore, another method has been developed in which the spark plug produces a constant electric arc over a 30 ° crank angle. In this case, up to 20 MJ of energy is released, which is much more than with a conventional spark discharge. It is known that if a sufficient amount of energy is not generated during spark ignition, the mixture will not ignite.
The plasma arc, in combination with the rotation of the charge in the combustion chamber, forms a large ignition surface, since during this the shape and size of the plasma arc change significantly. Along with an increase in the duration of the ignition period, this also means the presence of a high energy released for it.
Unlike standard system a constant voltage of 3000 V operates in the secondary circuit of the plasma ignition system. At the moment of discharge, a normal spark appears in the spark gap of the spark plug. In this case, the resistance at the electrodes of the candle decreases, and a constant voltage of 3000 V forms an arc, ignited at the moment of discharge. A voltage of about 900 V is sufficient to maintain the arc.
The plasma ignition system differs from the standard built-in high frequency (12 kHz) 12 V DC chopper. The induction coil raises the voltage to 3000 V, which is then rectified. It should be noted that prolonged arc discharge on a spark plug will significantly reduce its service life.
With plasma ignition, the flame spreads through the combustion chamber faster, so a corresponding change in the ignition timing is required. Tests of the plasma ignition system on a Ford Pinto car (USA) with an engine capacity of 2300 cm3 and an automatic transmission gave the results shown in Table. 12.
Table 12. Test results of the plasma ignition system on a Ford Pinto car
| Ignition system type | Emission of toxic substances, g | Fuel consumption, l / 100 km | |||
| CHx | CO | NOx |
urban test cycle | road test cycle |
|
| Standard | 0,172 | 3,48 | 1,12 | 15,35 | 11,41 |
| Plasma with optimal ignition timing | 0,160 | 3,17 | 1,16 | 14,26 | 10,90 |
| Plasma with optimal control of the ignition timing and mixture composition | 0,301 | 2,29 | 1,82 | 13,39 | 9,98 |
With plasma ignition, it is possible to carry out high-quality control of a gasoline engine, in which the amount of supplied air remains unchanged, and the engine power is controlled only by adjusting the amount of supplied fuel. When a plasma ignition system was used in the engine without changing the ignition timing and mixture composition, the fuel consumption decreased by 0.9%, when adjusting the ignition angle - by 4.5%, and with optimal regulation of the ignition angle and mixture composition - by 14% ( see Table 12). Plasma ignition improves engine performance, especially at partial loads, and fuel consumption can be the same as diesel.
REDUCING THE EMISSION OF TOXIC SUBSTANCES WITH EXHAUST GASES
The increase in motorization brings with it the need for environmental protection measures. The air in cities is more and more polluted by substances harmful to human health, especially carbon monoxide, unburned hydrocarbons, nitrogen oxides, compounds of lead, sulfur, etc. These are largely products of incomplete combustion of fuels used in enterprises, in everyday life, as well as in car engines.
Along with toxic substances during the operation of cars, their noise also has a harmful effect on the population. In recent years, the noise level in cities has increased by 1 dB annually, so it is necessary not only to stop the increase in the overall noise level, but also to achieve its reduction. The constant exposure to noise causes nervous diseases, reduces the ability to work of people, especially those engaged in mental activity. Motorization brings noise to previously quiet, remote locations. Unfortunately, the reduction of noise generated by woodworking and agricultural machines has not received due attention. The chainsaw creates noise in a large part of the forest, which causes changes in the living conditions of animals and often causes the disappearance of certain species.
Most often, however, the pollution of the atmosphere by the exhaust gases of cars causes criticism.
Table 13. Permissible emission of harmful substances with exhaust gases of passenger cars in accordance with the legislation pcs. California, USA
With busy traffic, exhaust gases accumulate at the surface of the soil and in the presence of solar radiation, especially in industrial cities located in poorly ventilated basins, so-called smog is formed. The atmosphere is polluted to such an extent that being in it is harmful to health. Road officials at some busy intersections use oxygen masks to maintain their health. Particularly harmful is the relatively heavy carbon monoxide located near the earth's surface, penetrating into the lower floors of buildings, garages and more than once leading to deaths.
Legislative enterprises limit the content of harmful substances in the exhaust gases of cars, and they are constantly being tightened (Table 13).
Regulations are a big concern for car manufacturers; they also indirectly affect the efficiency of road transport.
For complete combustion of the fuel, a certain excess of air can be allowed in order to ensure good mixing of the fuel with it. The required excess air depends on the degree of mixing of fuel with air. In carburetor engines, this process takes a long time, since the fuel path from the mixture-forming device to the spark plug is quite long.
The modern carburetor allows you to form different kinds mixtures. The richest mixture is needed for cold start of the engine, since a significant portion of the fuel condenses on the walls of the intake manifold and does not immediately enter the cylinder. Only a small portion of the light fuel fractions evaporates. When the engine warms up, a mixture of a rich composition is also required.
When the car is moving, the composition of the fuel-air mixture should be poor, which will provide good efficiency and low specific fuel consumption. To achieve maximum engine power, you need a rich mixture to fully utilize the entire mass of air entering the cylinder. To ensure good dynamic properties of the engine when the throttle valve is quickly opened, it is necessary to additionally supply a certain amount of fuel to the intake manifold, which compensates for the fuel that has settled and condensed on the walls of the pipeline as a result of an increase in pressure in it.
For good mixing of fuel with air, a high air speed and rotation should be created. If the cross-section of the carburetor diffuser is constant, then at low engine speeds for good mixture formation, the air speed in it is low, and at high speeds, the diffuser resistance leads to a decrease in the mass of air entering the engine. This disadvantage can be eliminated by using a variable cross-section carburetor or injecting fuel into the intake manifold.
There are several types of gasoline intake manifold injection systems. In the most commonly used systems, fuel is supplied through a separate nozzle for each cylinder, thereby achieving an even distribution of fuel between the cylinders, eliminating the settling and condensation of fuel on the cold walls of the intake manifold. The amount of injected fuel is easier to approach the optimum required by the engine in this moment... There is no need for a diffuser, and energy losses arising from the passage of air are eliminated. An example of such a fuel delivery system is the frequently used Bosch K-Jetronic injection system, already mentioned in 9.5 when considering turbocharged engines.
The diagram of this system is shown in Fig. 94. Tapered branch pipe / in which the swinging on the lever moves 2 valve 5 is designed so that the valve lift is proportional to the air mass flow. Window 5 for the passage of fuel are opened by a spool 6 in the regulator body when the lever is moved under the influence of the incoming air chute. The necessary changes in the composition of the mixture in accordance with the individual characteristics of the engine are achieved by the shape of the conical pipe. The lever with the valve is balanced by the counterweight, the inertia forces during vehicle vibrations do not affect the valve.
The air flow to the engine is regulated by the throttle valve 4. Damping of valve vibrations, and with it the spool, arising at low engine speeds due to pulsations of air pressure in the intake manifold, is achieved by jets in the fuel system. The screw 7 located in the valve lever also serves to regulate the amount of fuel supplied.
Between window 5 and nozzle 8 positioned control valve 10, spring-supported 13 and saddles 12, based on the membrane //, constant injection pressure in the "nozzle atomizer of 0.33 MPa at a pressure in front of the valve of 0.47 MPa.
Fuel from tank 16 supplied by an electric fuel pump 15 via pressure regulator 18 and fuel filter 17 into the lower chamber 9 regulator housing. Constant fuel pressure in the regulator is maintained by a pressure reducing valve 14. Diaphragm regulator 18 designed to maintain fuel pressure when the engine is not running. This prevents the formation of air pockets and ensures good starting of a hot engine. The regulator also slows down the increase in fuel pressure when the engine is started and dampens its fluctuations in the pipeline.
Several devices make it easier to start the engine cold. Bypass valve 20, controlled by a bimetallic spring, opens the drain line to the fuel tank during cold start, which reduces the fuel pressure on the end of the spool. This upsets the balance of the lever and the same amount of incoming air will correspond to a larger volume of injected fuel. Another device is an auxiliary air regulator 19, the diaphragm of which is also opened by a bimetallic spring. Additional air is needed to overcome the increased frictional resistance of a cold engine. The third device is a fuel injector 21 cold start, thermostat controlled 22 in the engine water jacket, which keeps the injector open until the engine coolant reaches the set temperature.
The electronic equipment of the considered gasoline injection system is limited to a minimum. The electric fuel pump is switched off when the engine is stopped and, for example, in an accident, the fuel supply is cut off, which prevents a fire in the car. When the engine is not running, a lever in the down position presses a switch located below it, which interrupts the current supplied to the starter and the thermostat heating coils. Cold start injector performance is dependent on engine temperature and engine running time.
If more air is supplied to one cylinder from the intake manifold than to the others, then the fuel supply is determined by the operating conditions of the cylinder with a large amount of air, i.e., with poor mixtureto ensure reliable ignition. In this case, the remaining cylinders will operate with enriched mixtures, which is economically disadvantageous and leads to an increase in the content of harmful substances.
In diesel engines, mixture formation is more difficult, since a very short time is spent on mixing fuel and air. The fuel ignition process begins with a slight delay after the start of fuel injection into the combustion chamber. During the combustion process, fuel injection is still in progress and under such conditions it is impossible to achieve full use of the air.
In diesel engines, therefore, there must be an excess of air and even when smoking (which indicates incomplete combustion of the mixture), unused oxygen is present in the exhaust gases. This is caused by poor mixing of fuel droplets with air. There is a lack of air in the center of the fuel flare, which leads to smoke, although there is unused air in the immediate vicinity of the flare. Some of this was already mentioned in 8.7.
The advantage of diesel engines is that ignition of the mixture is guaranteed even with a large excess of air. Failure to use the entire amount of air entering the cylinder during combustion is the reason for the relatively low specific power of the diesel engine per unit of weight and displacement, despite its high compression ratio.
More perfect mixture formation takes place in diesel engines with separated combustion chambers, in which the burning rich mixture from the additional chamber enters the main combustion chamber filled with air, mixes well with it and burns. This requires less excess air than with direct fuel injection, but the large cooling wall surface leads to large heat losses, which causes a drop in the indicator efficiency.
13.1. FORMATION OF CARBON OXIDE CO AND HYDROCARBONS CHx
When burning a mixture of stoichiometric composition, harmless carbon dioxide CO2 and water vapor should be formed, and if there is a lack of air due to the fact that part of the fuel burns incompletely, additionally toxic carbon monoxide CO and unburned hydrocarbons CHx.
These components of the exhaust gases which are harmful to health can be burned and rendered harmless. For this purpose, it is necessary to supply fresh air with a special compressor K (Fig. 95) to a place in the exhaust pipe where harmful products of incomplete combustion can be burned. Sometimes for this, air is supplied directly to the hot exhaust valve.
As a rule, a thermal reactor for afterburning CO and CHx is placed immediately behind the engine, directly at the outlet of the exhaust gases. Exhaust gases Msupplied to the center of the reactor, and removed from its periphery to the outlet pipeline V.The outer surface of the reactor has thermal insulation I.
In the most heated central part of the reactor, there is a fire chamber heated by exhaust gases,
where the products of incomplete combustion of fuel are burned. This releases heat, which keeps the reactor at a high temperature.
Unburned components in the exhaust gases can be oxidized without combustion using a catalyst. To do this, it is necessary to add secondary air to the exhaust gases, which is necessary for oxidation, the chemical reaction of which is carried out by a catalyst. This also releases heat. Usually rare and precious metals serve as a catalyst, so it is very expensive.
The catalysts can be used in any type of engine, but they have a relatively short service life. If lead is present in the fuel, then the catalyst surface is quickly poisoned, and it becomes unusable. Obtaining high-octane gasoline without lead antiknock agents is a rather complicated process in which a lot of oil is consumed, which is economically inexpedient when it is in short supply. It is clear that the afterburning of fuel in a thermal reactor leads to energy losses, although the combustion releases heat that can be utilized. Therefore, it is advisable to organize the process in the engine in such a way that when the fuel is burned in it, a minimum amount of harmful substances is formed. At the same time, it should be noted that the use of catalysts will be inevitable in order to fulfill the promising legislative requirements.
FORMATION OF NITROGEN OXIDES NOx
Nitrogen oxides, harmful to health, are formed at high combustion temperatures under conditions of a stoichiometric mixture. Reducing the emission of nitrogen compounds is associated with certain difficulties, since the conditions for their reduction coincide with the conditions for the formation of harmful products of incomplete combustion and vice versa. At the same time, the combustion temperature can be lowered by introducing some inert gas or water vapor into the mixture.
For this purpose, it is advisable to recirculate cooled exhaust gases into the intake manifold. The resulting decreasing power requires an enrichment of the mixture, a larger opening of the throttle valve, which increases the total emission of harmful CO and CHx with the exhaust gases.
Exhaust gas recirculation, together with reduced compression ratio, variable valve timing and later ignition, can reduce NOx by 80%.
Nitrogen oxides are removed from the exhaust gases using also catalytic methods. In this case, the exhaust gases are first passed through a reduction catalyst, in which the NOx content is reduced, and then, together with additional air, through an oxidation catalyst, where CO and CHx are eliminated. A diagram of such a two-component system is shown in Fig. 96.
To reduce the content of harmful substances in the exhaust gases, so-called β-probes are used, which can also be used in conjunction with a two-component catalyst. The peculiarity of the system with a probe is that the additional air for oxidation is not supplied to the catalyst, but the probe constantly monitors the oxygen content in the exhaust gases and controls the fuel supply so that the mixture is always stoichiometric. In this case, CO, CHx and NOx will be present in the exhaust gases in minimal amounts.
The principle of operation of the probe is that in a narrow range near the stoichiometric composition of the mixture \u003d 1, the voltage between the inner and outer surfaces of the probe changes sharply, which serves as a control impulse for the device that regulates the fuel supply. Sensing element 1 the probe is made of zirconium dioxide, and its surface 2 covered with a layer of platinum. The voltage characteristic Us between the inner and outer surfaces of the sensitive element is shown in Fig. 97.
OTHER TOXIC SUBSTANCES
Antiknock agents such as tetraethyl lead are usually used to increase the octane number of the fuel. To prevent lead compounds from settling on the walls of the combustion chamber and valves, so-called scavengers are used, in particular, dibromoethyl.
These compounds enter the atmosphere with exhaust gases and pollute the vegetation along the roads. Entering the human body with food, lead compounds adversely affect his health. Lead deposition in exhaust gas catalysts has already been mentioned. In this regard, an important task at the present time is the removal of lead from gasoline.
Oil entering the combustion chamber does not completely burn out, and the content of CO and CHx in the exhaust gases increases. To eliminate this phenomenon, high tightness of the piston rings and maintaining a good technical condition engine.
Combustion of large amounts of oil is especially common in two-stroke enginesin which it is added to the fuel. The negative consequences of using gas-oil mixtures are partially mitigated by metering oil with a special pump in accordance with the engine load. Similar difficulties exist when using the Wankel engine.
Gasoline vapors also have a harmful effect on human health. Therefore, ventilation of the crankcase must be carried out in such a way that gases and vapors that enter the crankcase due to poor tightness do not enter the atmosphere. Leakage of gasoline vapors from fuel tank can be prevented by adsorption and suction of vapors during intake system... Leakage of oil from the engine and gearbox, contamination of the car as a result of this with oils is also prohibited in order to maintain a clean environment.
Reducing oil consumption is just as important from an economic point of view as saving fuel because oils are significantly more expensive than fuel. Conducting regular monitoring and maintenance reduce oil consumption due to engine malfunctions. Oil leaks in the engine can be observed, for example, due to poor tightness of the cylinder head cover. Oil leaks can contaminate the engine and cause a fire.
Oil leakage is also unsafe due to the poor tightness of the crankshaft seal. In this case, oil consumption increases noticeably, and the car leaves dirty marks on the road.
Oil contamination of a car is very dangerous, and oil stains under the car serve as an excuse to prohibit its operation.
Oil leaking through the crankshaft seal can enter the clutch and cause it to slip. However, more negative consequences are caused by the ingress of oil into the combustion chamber. And although the oil consumption is relatively low, but its incomplete combustion increases the emission of harmful components with the exhaust gases. Burning oil manifests itself in excessive smoke from the car, which is typical for two-stroke, as well as significantly worn four-stroke engines.
IN four-stroke engines oil enters the combustion chamber through the piston rings, which is especially noticeable when they and the cylinder are heavily worn. The main reason for oil penetration into the combustion chamber is the uneven fit of the compression rings to the cylinder circumference. Oil is drained from the cylinder walls through the oil scraper ring slots and holes in its groove.
Through the gap between the stem and the intake valve guide, oil easily enters the intake manifold where there is a vacuum. This is especially true when using low viscosity oils. Oil flow through this assembly can be prevented by using a rubber gland on the end of the valve guide.
The crankcase gases of the engine, which contain many harmful substances, are usually discharged by a special pipeline into the intake system. Passing from it to the cylinder, crankcase gases burn together with the air-fuel mixture.
Low viscosity oils reduce friction losses, improve mechanical efficiency of the engine and reduce fuel consumption. However, it is not recommended to use oils with a viscosity lower than prescribed by the standards. This can cause increased oil consumption and high engine wear.
Due to the need to save oil, the collection and use of waste oil is becoming an increasingly important issue. By reclaiming old oils, a significant amount of quality liquid lubricants can be obtained while simultaneously preventing environmental pollution by stopping the discharge of used oils into water streams.
DETERMINATION OF THE PERMISSIBLE AMOUNT OF HARMFUL SUBSTANCES
Elimination of harmful substances from exhaust gases is a rather difficult task. In high concentrations, these components are very harmful to health. Of course, it is impossible to immediately change the current situation, especially in relation to the exploited car park. For this reason, the statutory regulations for the control of harmful substances in exhaust gases have been designed for new vehicles being produced. These prescriptions will be gradually improved taking into account new advances in science and technology.
Exhaust gas cleaning is associated with an increase in fuel consumption by almost 10%, a decrease in engine power and an increase in the cost of the car. At the same time, the cost of vehicle maintenance also increases. Catalysts are also expensive as their components are composed of rare metals. The service life should be calculated for 80,000 km of the vehicle's mileage, but it has not yet been reached. The catalysts currently in use last about 40,000 km using lead-free gasoline.
The current situation calls into question the effectiveness of strict regulations on the content of harmful impurities, since this causes a significant increase in the cost of the car and its operation, and also leads to an increase in oil consumption as a result.
Fulfillment of the stringent requirements for the purity of exhaust gases put forward in the future with the current state of gasoline and diesel engines is not yet possible. Therefore, it is advisable to pay attention to a radical change in the power plant of motor vehicles.
According to Carnot's theory, we are obliged to transfer part of the heat energy supplied to the cycle to the environment, and this part depends on the temperature difference between hot and cold heat sources.
The secret of the turtle
A feature of all heat engines obeying the Carnot theory is the use of the process of expansion of the working fluid, which makes it possible to obtain mechanical work in the cylinders of piston engines and in the rotors of turbines. The pinnacle of today's heat and power engineering in terms of the efficiency of converting heat into work are combined-cycle plants. In them, the efficiency exceeds 60%, with temperature differences over 1000 ºС.
In experimental biology, more than 50 years ago, amazing facts were established that contradicted the well-established concepts of classical thermodynamics. Thus, the efficiency of the turtle's muscular activity reaches an efficiency of 75-80%. In this case, the temperature difference in the cell does not exceed fractions of a degree. Moreover, both in a heat engine and in a cell, the energy of chemical bonds is first converted into heat in oxidation reactions, and then heat is converted into mechanical work. Thermodynamics on this matter prefers to be silent. According to its canons, for such an efficiency, temperature differences are needed that are incompatible with life. What is the secret of the turtle?
Traditional processes
From the days of Watt's steam engine, the first mass-produced heat engine, to the present day, the theory of heat engines and technical solutions on their implementation have gone a long way of evolution. This direction gave rise to a huge number of design developments and related physical processes, the general task of which was the conversion of thermal energy into mechanical work. The concept of "compensation for the conversion of heat into work" was unchanged for the whole variety of heat engines. This concept is perceived today as absolute knowledge, daily proven by all known practice of human activity. Note that the facts of known practice are not at all the base of absolute knowledge, but only the knowledge base of this practice. For example, planes did not always fly.
A common technological disadvantage of today's heat engines (internal combustion engines, gas and steam turbines, rocket engines) is the need to transfer to the environment most of the heat supplied to the heat engine cycle. This is mainly why they have low efficiency and economy.
Let's pay special attention to the fact that all of the listed heat engines use the processes of expansion of the working fluid to convert heat into work. It is these processes that make it possible to convert the potential energy of the thermal system into the cooperative kinetic energy of the flows of the working fluid and then into the mechanical energy of the moving parts of heat machines (pistons and rotors).
Let us note one more, albeit trivial, fact that heat engines operate in an air atmosphere under the constant compression of gravitational forces. It is the forces of gravity that create the pressure of the environment. Compensation for converting heat into work is associated with the need to perform work against the forces of gravity (or, equivalently, against the environmental pressure caused by the forces of gravity). The combination of the two above-mentioned facts leads to the "inferiority" of all modern heat engines, to the need to transfer part of the heat supplied to the cycle to the environment.
The nature of compensation
The nature of the compensation for the conversion of heat into work is that 1 kg of the working fluid at the exit from the heat engine has a larger volume - under the influence of expansion processes inside the machine - than the volume at the entrance to the heat engine.
This means that by driving 1 kg of the working fluid through the heat engine, we expand the atmosphere by an amount, for which it is necessary to perform work against the forces of gravity - the work of pushing through.
Part of the mechanical energy received in the machine is spent on this. However, pushing work is only one part of the compensation energy cost. The second part of the costs is associated with the fact that 1 kg of the working fluid at the exhaust from the heat engine into the atmosphere must have the same atmospheric pressure as at the inlet to the machine, but with a larger volume. And for this, in accordance with the equation of the gaseous state, it must also have a higher temperature, that is, we are forced to transfer additional internal energy to a kilogram of a working fluid in a heat engine. This is the second component of compensation for converting heat into work.
The nature of compensation is formed from these two components. Let's pay attention to the interdependence of the two compensation components. The greater the volume of the working fluid at the exhaust from the heat engine compared to the volume at the inlet, the greater is not only the work to expand the atmosphere, but also the necessary increase in internal energy, i.e., the heating of the working fluid at the exhaust. And vice versa, if, due to regeneration, the temperature of the working fluid at the exhaust is reduced, then, in accordance with the equation of the gas state, the volume of the working fluid, and hence the work of pushing, will also decrease. If we carry out deep regeneration and reduce the temperature of the working fluid at the exhaust to the temperature at the inlet and thereby simultaneously equalize the volume of a kilogram of the working fluid at the exhaust to the volume at the inlet, then the compensation for the conversion of heat into work will be zero.
But there is a fundamentally different way of converting heat into work, without using the process of expanding the working fluid. In this method, an incompressible liquid is used as a working fluid. The specific volume of the working fluid in the cyclic process of converting heat into work remains constant. For this reason, there is no expansion of the atmosphere and, accordingly, no energy consumption, typical of heat engines using expansion processes. There is no need to compensate for the conversion of heat into work. This is possible in the bellows. The supply of heat to a constant volume of incompressible fluid leads to a sharp increase in pressure. So, heating water at a constant volume by 1 ºС leads to an increase in pressure by five atmospheres. This effect is used to change the shape (we have compression) of the bellows and perform work.
Bellows piston engine
The heat engine proposed for consideration implements the aforementioned fundamentally different way of converting heat into work. This installation, excluding the transfer of most of the supplied heat to the environment, does not need compensation for converting heat into work.
To realize these possibilities, a heat engine is proposed, which contains working cylinders, the inner cavity of which is united by means of a bypass pipeline with control valves. It is filled as a working fluid with boiling water (wet steam with a dryness degree of the order of 0.05-0.1). Bellows pistons are located inside the working cylinders, the inner cavity of which is united by means of a bypass pipeline into a single volume. The inner cavity of the bellows pistons is connected to the atmosphere, which ensures constant atmospheric pressure inside the volume of the bellows.
The bellows pistons are connected by a slider with a crank mechanism, which converts the traction force of the bellows pistons into the rotational motion of the crankshaft.
The working cylinders are located in the volume of the vessel filled with boiling transformer or turbine oil. Oil boiling in the vessel is provided by the supply of heat from an external source. Each working cylinder has a removable heat-insulating casing, which, at the right time, either covers the cylinder, stopping the heat transfer process between the boiling oil and the cylinder, or frees the surface of the working cylinder and at the same time ensures the transfer of heat from the boiling oil to the working fluid of the cylinder.
The shells are divided along the length into separate cylindrical sections, consisting of two halves, shells, when approaching, covering the cylinder. A design feature is the arrangement of the working cylinders along one axis. The rod provides mechanical interaction between the bellows pistons of different cylinders.
The bellows piston, made in the form of a bellows, is fixedly fixed on one side with a pipeline connecting the inner cavities of the bellows pistons with the dividing wall of the working cylinder body. The other side, attached to the slider, is movable and moves (compressed) in the inner cavity of the working cylinder under the influence of the increased pressure of the working body of the cylinder.
A bellows is a thin-walled corrugated tube or chamber made of steel, brass, bronze, stretching or compressing (like a spring) depending on the difference in pressure inside and outside or on an external force.
The bellows piston, on the other hand, is made of non-thermally conductive material. It is possible to manufacture the piston from the above-mentioned materials, but covered with a non-thermally conductive layer. The piston also has no spring properties. Its compression occurs only under the influence of the pressure difference along the sides of the bellows, and extension - under the influence of the rod.
Engine operation
The heat engine works as follows.
We will begin the description of the operating cycle of a heat engine with the situation shown in the figure. The bellows piston of the first cylinder is fully extended and the bellows piston of the second cylinder is fully compressed. The heat-insulating casings on the cylinders are firmly pressed against them. The valve on the pipeline connecting the inner cavities of the working cylinders is closed. The temperature of the oil in the oil container in which the cylinders are located is brought to a boil. The pressure of boiling oil in the cavity of the vessel, the working fluid inside the cavities of the working cylinders, is equal to atmospheric. The pressure inside the cavities of the bellows pistons is always equal to atmospheric - since they are connected to the atmosphere.
The state of the working fluid of the cylinders corresponds to point 1. At this moment, the fittings and the heat-insulating casing on the first cylinder open. The shells of the heat-insulating casing move away from the shell surface of the cylinder 1. In this state, heat transfer from the boiling oil in the vessel in which the cylinders are located to the working fluid of the first cylinder is ensured. On the other hand, the heat-insulating jacket on the second cylinder tightly fits the surface of the cylinder shell. The shells of the heat-insulating casing are pressed against the surface of the shell of cylinder 2. Thus, the transfer of heat from the boiling oil to the working fluid of cylinder 2 is impossible. Since the temperature of oil boiling at atmospheric pressure (about 350 ºС) in the cavity of the vessel containing the cylinders is higher than the temperature of water boiling at atmospheric pressure (wet steam with a dryness degree of 0.05-0.1) in the cavity of the first cylinder, intensive transfer of thermal energy from boiling oil to the working fluid (boiling water) of the first cylinder.
How the work is done
During the operation of a bellows-piston engine, a significantly harmful moment appears.
Heat is transferred from working area bellows accordion, where the conversion of heat into mechanical work is carried out, into the non-working zone during the cyclic movement of the working fluid. This is unacceptable, since heating the working fluid outside the working area leads to a pressure drop across the inoperative bellows. Thus, a harmful force will arise against the production of useful work.
Losses from cooling the working fluid in a bellows-piston engine are not as fundamentally inevitable as heat losses in Carnot's theory for cycles with expansion processes. Cooling losses in a bellows piston engine can be reduced to an arbitrarily small value. Note that in this work we are talking about thermal efficiency. The internal relative efficiency associated with friction and other technical losses remains at the level of today's engines.
There can be any number of paired working cylinders in the described heat engine, depending on the required power and other design conditions.
At small temperature differences
In the nature around us, there are constantly various temperature drops.
For example, temperature differences between water layers of different heights in the seas and oceans, between water and air masses, temperature drops near thermal springs, etc. Let us show the possibility of a bellows-piston engine operating at natural temperature drops, using renewable energy sources. Let's make estimates for the climatic conditions of the Arctic.
The cold layer of water starts from the lower edge of the ice, where its temperature is 0 ° С and up to + 4-5 ° С. In this area, we will remove that small amount of heat that is taken from the bypass pipeline to maintain a constant temperature level of the working fluid in the non-working zones of the cylinders. For a circuit (heat conduit) that removes heat, we select butylene cis-2-B as the heat carrier (boiling-condensation temperature at atmospheric pressure is +3.7 ° C) or butyne 1-B (boiling point + 8.1 ° C) ... The warm water layer in depth is determined in the temperature range 10-15 ° С. Here we lower the bellows-piston engine. The working cylinders are in direct contact with sea water. As the working fluid of the cylinders, we select substances that have a boiling point at atmospheric pressure below the temperature of the warm layer. This is necessary to ensure heat transfer from seawater to the working fluid of the engine. Boron chloride (boiling point +12.5 ° C), butadiene 1.2 ‑ B (boiling point +10.85 ° C), vinyl ether (boiling point +12 ° C) can be offered as the working fluid of the cylinders.
There are a large number of inorganic and organic substances that meet these conditions. Heating circuits with such selected heat carriers will operate in the heat pipe mode (in the boiling mode), which will ensure the transfer of high heat capacities with small temperature drops. The pressure drop between the outer side and the inner cavity of the bellows, multiplied by the area of \u200b\u200bthe bellows accordion, creates a force on the slide and generates engine power proportional to the power supplied to the cylinder by heat.
If the heating temperature of the working fluid is reduced tenfold (by 0.1 ° C), then the pressure drop along the sides of the bellows will also decrease by about ten times, to 0.5 atmospheres. If at the same time the area of \u200b\u200bthe bellows accordion is also increased tenfold (increasing the number of bellows sections), then the force on the slide and the developed power will remain unchanged with a constant supply of heat to the cylinder. This will allow, firstly, to use very small natural temperature drops and, secondly, to drastically reduce the harmful heating of the working fluid and heat removal into the environment, which will make it possible to obtain high efficiency. Although there is a desire for high. Estimates show that the engine power at natural temperature changes can be up to several tens of kilowatts per square meter of the heat-conducting surface of the working cylinder. In the considered cycle, there are no high temperatures and pressures, which significantly reduces the cost of the installation. The engine, when operating at natural temperature changes, does not emit harmful emissions into the environment.
As a conclusion, the author would like to say the following. The postulate of "compensation for the transformation of heat into work" and the irreconcilable position of the carriers of these delusions, far beyond the scope of polemical decency, tied creative engineering thought, gave rise to a tight knot of problems. It should be noted that engineers have long invented a bellows and it is widely used in automation as a power element that converts heat into work. But the current situation in thermodynamics does not allow for an objective theoretical and experimental study of its work.
The disclosure of the nature of the technological shortcomings of modern heat engines showed that "compensation for the conversion of heat into work" in its established interpretation and the problems and negative consequences that he faced for this reason modern world, is nothing more than compensation for incomplete knowledge.
In the engine cylinder, thermodynamic cycles are carried out with some frequency, which are accompanied by a continuous change in the thermodynamic parameters of the working fluid - pressure, volume, temperature. The energy of fuel combustion with a change in volume turns into mechanical work. The condition for the transformation of heat into mechanical work is a sequence of strokes. These strokes in an internal combustion engine include intake (filling) of cylinders with a combustible mixture or air, compression, combustion, expansion, and exhaust. The variable volume is the volume of the cylinder, which increases (decreases) with the translational movement of the piston. An increase in volume occurs due to the expansion of products during the combustion of a combustible mixture, a decrease - when a new charge of a combustible mixture or air is compressed. The forces of gas pressure on the cylinder walls and on the piston during the expansion stroke are transformed into mechanical work.
The energy stored in the fuel is converted into thermal energy during thermodynamic cycles, is transferred to the cylinder walls by thermal and light radiation, radiation and from the cylinder walls - the coolant and the engine mass by thermal conduction and into the surrounding space from the engine surfaces free and forced
convection. All types of heat transfer are present in the engine, which indicates the complexity of the processes taking place.
The use of heat in the engine is characterized by efficiency, the less the heat of combustion of fuel is given to the cooling system and to the engine mass, the more work is done and the higher the efficiency.
The engine runs in two or four strokes. The main processes of each working cycle are intake, compression, stroke and exhaust strokes. The introduction of a compression stroke into the working process of engines made it possible to minimize the cooling surface and at the same time increase the fuel combustion pressure. Combustion products expand according to the compression of the combustible mixture. This process allows to reduce heat losses in the cylinder walls and with exhaust gases, to increase the gas pressure on the piston, which significantly increases the power and economic performance of the engine.
Real thermal processes in an engine differ significantly from theoretical ones based on the laws of thermodynamics. The theoretical thermodynamic cycle is closed, a prerequisite for its implementation is the transfer of heat to a cold body. In accordance with the second law of thermodynamics and in a theoretical heat engine, it is impossible to completely convert thermal energy into mechanical energy. In diesel engines, the cylinders of which are filled with a fresh charge of air and have high compression ratios, the temperature of the combustible mixture at the end of the intake stroke is 310 ... 350 K, which is explained by the relatively small amount of residual gases; in gasoline engines, the intake temperature at the end of the stroke is 340 .. .400 K. The heat balance of the combustible mixture during the intake stroke can be represented as
where?) p t - the amount of heat of the working fluid at the beginning of the intake stroke; Os.ts - the amount of heat that entered the working fluid upon contact with the heated surfaces of the intake tract and cylinder; Qo g - the amount of heat in the residual gases.
From the heat balance equation, the temperature at the end of the intake stroke can be determined. We take the mass value of the amount of fresh charge t with z, residual gases - t about g With a known heat capacity of the fresh charge with P, residual gases with "p and working mixture with p equation (2.34) is represented as
where T with h - temperature of the fresh charge before inlet; AND T sz - heating of a fresh charge when it is injected into the cylinder; T g - temperature of residual gases at the end of the discharge. It is possible to assume with sufficient accuracy that with "p = with p and s "p - s, s p, where s; - correction factor depending on T sz and the composition of the mixture. With a \u003d 1.8 and diesel fuel
When solving equation (2.35) with respect to T a denote the relation 
The formula for determining the temperature in the cylinder at the inlet has the form
This formula is valid for both four-stroke and two-stroke engines; for turbocharged engines, the temperature at the end of the intake is calculated by formula (2.36), provided that q \u003d 1. The accepted condition does not introduce large errors into the calculation. The values \u200b\u200bof the parameters at the end of the intake stroke, determined experimentally at the nominal mode, are presented in table. 2.2.
Table 2.2
|
Four-stroke ICE |
Two-stroke internal combustion engines |
||
|
Index |
spark ignition |
with direct flow gas exchange |
|
|
Residual gas coefficient at ost |
|||
|
Exhaust gas temperature at the end of the exhaust G p K |
|||
|
Heating of fresh charge, K |
|||
|
Working fluid temperature at the end of the inlet T a, TO |
|||
During the intake stroke, the intake valve in the diesel engine opens by 20 ... 30 ° before the piston reaches TDC and closes after passing the BDC by 40 ... 60 °. The opening time of the inlet valve is 240 ... 290 °. The temperature in the cylinder at the end of the previous stroke - exhaust is equal to T g \u003d 600 ... 900 K. The air charge, which has a temperature significantly lower, is mixed with the residual gases in the cylinder, which reduces the temperature in the cylinder at the end of the intake to T a \u003d 310 ... 350 K. The temperature difference in the cylinder between the exhaust and intake strokes is AT a. r \u003d T a - T g.Because the T a AT a. t \u003d 290 ... 550 °.
The rate of temperature change in the cylinder per unit of time per cycle is equal to:
For a diesel engine, the rate of temperature change during the intake stroke at n e \u003d 2400 min -1 and φ a \u003d 260 ° is with d \u003d (2.9 ... 3.9) 10 4 deg / s. Thus, the temperature at the end of the intake stroke in the cylinder is determined by the mass and temperature of the residual gases after the exhaust stroke and the heating of the fresh charge from the engine parts. The graphs of the function co rt \u003d / (D e) of the intake stroke for diesel and gasoline engines, presented in Fig. 2.13 and 2.14, indicate a significantly higher rate of temperature change in the cylinder of a gasoline engine in comparison with a diesel engine and, therefore, a higher intensity of the heat flow from the working fluid and its growth with an increase in the crankshaft speed. The average calculated value of the rate of temperature change during the diesel intake stroke within the crankshaft speed of 1500 ... 2500 min -1 is \u003d 2.3 10 4 ± 0.18 deg / s, and for the gasoline
engine within the speed of 2000 ... 6000 min -1 - with i \u003d 4.38 10 4 ± 0.16 deg / s. At the intake stroke, the temperature of the working fluid is approximately equal to the operating temperature of the coolant,
Figure: 2.13.
Figure: 2.14.
the heat of the cylinder walls is spent on heating the working fluid and does not significantly affect the temperature of the coolant in the cooling system.
When compression stroke happen enough complex processes heat transfer inside the cylinder. At the beginning of the compression stroke, the temperature of the charge of the combustible mixture is less than the temperature of the surfaces of the walls of the cylinder and the charge heats up, continuing to take heat from the walls of the cylinder. The mechanical work of compression is accompanied by the absorption of heat from the external environment. In a certain (infinitely small) period of time, the temperatures of the surface of the cylinder and the charge of the mixture equalize, as a result of which the heat transfer between them stops. With further compression, the temperature of the charge of the combustible mixture exceeds the temperature of the surfaces of the cylinder walls and the heat flux changes direction, i.e. heat goes to the cylinder walls. The total heat transfer from the charge of the combustible mixture is insignificant, it is about 1.0 ... 1.5% of the amount of heat supplied with the fuel.
The temperature of the working fluid at the end of the inlet and its temperature at the end of compression are related by the equation of the compression polytrope:
where 8 is the compression ratio; n l - polytropic indicator.
The temperature at the end of the compression stroke, as a general rule, is calculated by the average constant for the entire process value of the polytropic index n. In a particular case, the polytropic exponent is calculated from the balance of heat during compression in the form
where and with and and "- internal energy of 1 kmole of fresh charge; and a and and "-internal energy of 1 kmol of residual gases.
Joint solution of equations (2.37) and (2.39) at a known temperature T a allows to determine the polytropic index n. The polytropic index is influenced by the intensity of cylinder cooling. At low coolant temperatures, the cylinder surface temperature is lower, therefore n l will be less.
The values \u200b\u200bof the parameters of the end of the compression stroke are given in table. 2.3.
Table23
On the compression stroke, the intake and exhaust valves are closed, the piston moves to TDC. The time of the compression stroke for diesel engines at a speed of 1500 ... 2400 min -1 is 1.49 1СГ 2 ... 9.31 KG 3 s, which corresponds to the rotation of the crankshaft at an angle φ (. \u003d 134 °, for gasoline engines at a rotational speed of 2400 ... 5600 min -1 and cf r \u003d 116 ° - (3.45 ... 8.06) 1 (G 4 s. The temperature difference of the working fluid in the cylinder between the compression and intake strokes AT s _ a = T s - T a for diesel engines it is within 390 ... 550 ° С, for gasoline engines - 280 ... 370 ° С.
The rate of temperature change in the cylinder per compression stroke is:
and for diesel engines at a speed of 1500 ... 2500 min -1 the rate of temperature change is (3.3 ... 5.5) 104 deg / s, for gasoline engines at a speed of 2000 ... 6000 min -1 - ( 3.2 ... 9.5) x x 10 4 deg / s. The heat flow during the compression stroke is directed from the working fluid in the cylinder to the walls and into the coolant. Function graphs with \u003d f (n e) for diesel and gasoline engines are shown in Fig. 2.13 and 2.14. It follows from them that the rate of change in the temperature of the working fluid in diesel engines is higher than in gasoline engines at one speed.
Heat transfer processes during the compression stroke are determined by the temperature difference between the cylinder surface and the charge of the combustible mixture, the relatively small cylinder surface at the end of the stroke, the mass of the combustible mixture and a limited short period of time during which heat transfer from the combustible mixture to the cylinder surface occurs. It is assumed that the compression stroke has no significant effect on the temperature regime of the cooling system.
Expansion cycle is the only stroke in the engine's operating cycle during which useful mechanical work is performed. This cycle is preceded by the combustion process of the combustible mixture. The result of combustion is an increase in the internal energy of the working fluid, which is converted into work of expansion.
The combustion process is a complex of physical and chemical phenomena of fuel oxidation with intense release
warmth. For liquid hydrocarbon fuels (gasoline, diesel fuel), the combustion process is a chemical reaction of the combination of carbon and hydrogen with atmospheric oxygen. The heat of combustion of the charge of the combustible mixture is spent on heating the working fluid and performing mechanical work. Part of the heat from the working fluid through the cylinder walls and the head heats the crankcase and other engine parts, as well as the coolant. The thermodynamic process of a real working process, taking into account the loss of the heat of combustion of fuel, taking into account incomplete combustion, heat transfer to the cylinder walls, etc., is extremely complex. In diesel and gasoline engines, the combustion process is different and has its own characteristics. In diesel engines, combustion occurs with different intensities depending on the piston stroke: at first intensively, and then slowly. In gasoline engines, combustion occurs instantly, it is generally accepted that it occurs at a constant volume.
To take into account the heat by the components of losses, including heat transfer to the cylinder walls, the coefficient of utilization of the combustion heat is introduced.The coefficient of utilization of heat is determined experimentally, for diesel engines \u003d 0.70 ... 0.85 and gasoline engines ?, \u003d 0.85 ... 0.90 from the equation of state of gases at the beginning and end of expansion:
where is the degree of preliminary expansion.
For diesel engines
then 
For gasoline engines 
then
Parameter values \u200b\u200bduring combustion and at the end of the expansion stroke for engines)
